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Hard Disk and Solid State Drive Problems and Their Symptoms

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In this article, we will be describing problems with both traditional electromechanical hard disk drives (HDDs), solid state drives (SSDs), and external drives. If problems of the types discussed in this article are detected, the problems are often serious. Evaluating them properly may require a diagnostic tool to properly isolate the actual cause or severity of the problem.

This page is one page in a series of five pages dealing with HDD and SSD problems and their associated symptoms. If you haven't read the main page of this article, we recommend clicking on the link to the left titled "Article Main Page" before proceeding. This article will focus solely on problems specific to HDDs, SSDs, and external drives, and will not address any of the problems associated with supporting circuitry on the logic board, software, or other possible symptoms and problems.

Symptoms of Genuine Drive Problems

This article will address the list of symptoms that was defined in the main page of this article associated with actual drive problems. Each of the following subsections will describe the symptom, its possible causes, and how to correct it if possible  At the end of the article we will address symptoms and problems unique to external drives. Remember that an external drive whether its media is an HDD or SSD can potentially exhibit any of the symptoms associated with that specific type of drive in use (HDD or SSD) as well as other problems unique to external drives.

For the sake of review and clarity, the symptoms we identified in the main page of this article were in the classification titled A genuine drive problem and are as follows:


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Abnormal Noises

Abnormal noises are almost always a sign of a failing electromechanical hard drive (HDD), provided the noises are actually originating from the drive and they're abnormal. SSDs are totally electronic and cannot make noise unless a component, such as a capacitor or other circuit board component is literally in the process of failing catastrophically, which is highly unlikely. Most computers use cooling fans, and if the bearings fail or seize up, the noises they generate may sound nearly identical to those of a failing hard drive. Other devices, such as optical drives may cause noises that my delude a user into believing their hard drive has problems when it doesn't. Some units have shielding that may work its way loose and start making some noises that can sound similar to an HDD that's mechanically seizing. If abnormal noises can be traced to the hard drive, it's most likely in the process of failing, if it hasn't failed already. If you're in the process of troubleshooting a noise that you believe is coming from an  electromechanical hard drive, this needs to be confirmed.  Once again, SSDs generally will not make any noise.

If abnormal noises are confirmed to be coming from the drive, you should back up its contents as soon as possible.

Common noises coming from an HDD experiencing problems can include  the following:

Most of the noises above are a clear indicator of a drive that's about to fail, or beginning the initial stages of catastrophic failure. The exception to this is the "chug-a-chug" sound, which usually indicates a bad or weak sector, which doesn't necessarily mean the drive is about to fail, but has surface platter damage. We recommend becoming familiar with the noises coming from a mechanical hard drive when it's new so you'll be able to distinguish the normal from the abnormal. It is not uncommon for a drive to become louder as it ages. All drives make noise, but newer drives are nearly silent. If you listen to a drive in good working order operating normally it may make a faint noise, similar to the sound of rattling, with intermittent light clicking noises, with both being somewhat random in nature.

Excessively loud clicking noises are usually indicative of catastrophic failure, particularly if they're repetitive. This usually indicates that the drive heads are slamming against the extremes of the drive mechanism. In most cases, this is a catastrophic failure of the hard drive. This type of noise most often comes on suddenly, and it will be much louder than the light clicking noises you may hear from a newer drive in good working order.

Screeches and grinding noises, likewise, are usually signs of impending catastrophic failure. These types of noises may initially start appearing in short bursts that become increasingly more common as time progresses, until the drive eventually totally fails. The causes can be numerous, with none of them good, and include failing bearings, failing motors or actuators, or in some cases the dragging of the drive heads across the surface of a hard drive platter. If you have a drive in this type of condition, we recommend getting data off the drive as soon as possible. The drive may fail in a matter of minutes, hours, or even days, but in nearly all cases, failure is imminent.

The "chug-a-chug" noise is usually caused on a mechanical hard drive by a bad or weak sector. On some contemporary hard drives, which are extremely quiet compared to drives ten or more years older, this noise may not be immediately evident. Delays will almost always accompany these, possibly with the appearance of a "spinning beach ball." The noise itself, if audible, is caused in mechanical hard drives by the drive continually repositioning itself at the start of a bad or weak sector, attempting to read it, and when it can't be read, it repositions the drive heads to the start of the bad sector and starts over again. A bad or failed sector will never succeed in this operation, whereas a weak sector will eventually be able to be read. More information is provided below in the section on delays caused by bad or weak sectors.


With the exception of noise associated with bad or weak sectors, if any of other the noises just addressed are evident, the drive is failing and needs to be replaced. A drive in this condition may last anywhere from a few hours to a few weeks or longer, depending on use,  but it will almost certainly fail. Once again, SSDs will not make any noise whatsoever. In many cases bad or weak sectors can be dealt with, which will be covered below.

Delays From Bad or Weak Sectors

On electromechanical hard drives, delays from bad or weak sectors indicate damage to the platters of a hard drive, usually after a head crash. The only way to positively detect a bad or weak sector is by using a diagnostic tool to evaluate the drive. If damage is from a head crash, it is not uncommon for bad, weak, and good sectors do be dispersed in a localized but damaged region of the drive platters. Bad and weak sectors may also be the result of media defects or the platters losing their ability to retain information.

Under normal conditions, as the drive heads sweep over the platters, data is written and read by the drive heads along with error checking information. If bad or weak sectors exist on a drive, how they are handled by the drive controller depends on whether the operation is a read or write operation.

When data is being read, if the error checking succeeds, the drive heads move to the next location on the drive to perform more operations as needed. If  the error checking fails, the drive controller will re-issue the read command over and over again, which is time consuming. If the drive controller can eventually get the data in the sector to match its error checking information, the read operation will be a success, even though it might take anywhere from milliseconds to tens of seconds for it to obtain the information properly.  This is a weak sector - the data can be recovered but only after numerous retries. If, on the other hand, after a given amount of time or number of retries the data cannot be properly retrieved, the drive controller issues an I/O error and the operating system usually indicates that the operation failed. This is a bad sector - the data cannot be recovered, or at least it can't be recovered by the drive using its built in algorithms. Both of these will cause delays reading data from the drive, but only a bad sector will issue an I/O error.

During a write operation, if a bad sector is detected, the write operation will fail and the drive controller will attempt to re-map the bad sector out of the drive's available sectors and replace it with a spare sector. All contemporary hard drives have a number of spare sectors on the drive, and if some are still available, the operation will succeed. The user will likely never know it happened. If a head crash is severe enough, the supply of spare sectors will become depleted and the controller will not be able to map bad sectors out any longer. In this case, how the drive controller handles the situation may vary considerably from drive to drive. Some will just move on to another sector leaving the bad sector alone but still accessible, until it finds a good one. Other drives may completely lock up forcing the system to be rebooted, and the event will re-occur when the drive accesses the same bad sector.

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During a write operation, if a weak sector is detected, the write operation may or may not attempt to re-map the sector. Whether a re-map occurs is totally dependent on the drive controller's firmware and how tolerant it will be of weak sectors. Some controllers will allow a sector to remain in use even though it may take several seconds to read it. Although several seconds doesn't sound like much, if the sector is in a file that's read continually, such as a web browser's cache file, the delays may quickly add up to very long periods of time. A weak sector will not generate an I/O error either during a read or write operation.

Both bad and weak sectors typically cause delays because of the amount of time it takes to attempt to read them after a head crash has damaged existing data on a drive. Typically, a head crash spans a fairly large number of contiguous sectors, and as stated before, the sectors may consist of many blocks of bad, weak, or good sectors interspersed. When operating systems write data to a hard drive, most do so in contiguous blocks to minimize the lateral movement of the drive heads to increase performance. Unfortunately, this also means that after a head crash the damage to a file is sequential as well. As a result, during a read operation the controller may encounter many weak and bad sectors while attempting to read a single file, and the delays add up and are noticeable.

It should be noted that if bad or weak sectors are detected on regions of the drive during a read operation that contain (or used to contain) valid data, drives typically will not attempt to correct them. Correction of bad or weak sectors (if done) will always occur only during write operations.

generally cannot develop weak sectors, but they can develop bad blocks - in fact it's normal for them to do so. An SSD is totally electronic and totally digital. Unlike a mechanical hard drive that's converting an analog signal from the surface of a rotating platter to a digital format,  an SSD stores it's data in digital format. The media on an electromechanical hard drive, assuming it's undamaged, can be used to read and write data countless times until the media itself begins to fail to retain information, which is often on the order of a decade or longer. The memory cells on an SSD have a limited write cycle count, meaning that after they've been subjected to a certain number of writes, they fail and need to be replaced by spare blocks, similar to the spare sectors on a mechanical hard drive. As you might guess, the number of spare sectors on a mechanical hard drive is small compared to the number of spare blocks available on an SSD.

Ideally, the way an SSD handles block management is by monitoring write counts and data retention capability when writing data. If the write count is high and the ability to retain information appears to be marginal, the block is excluded from use and replaced by a spare block. Failing blocks will be re-mapped until the set of spares is exhausted. According to manufacturers, this typically occurs in terms of tens of years, not years.

Unfortunately, some blocks on SSDs simply refuse to behave like they're supposed to. They will simply go bad, for what appears to be no obvious or apparent reason. The problem is similar to a bad sector on a hard drive caused by a media defect. As with mechanical HDDs, the problem is somewhat of a rarity, but when it occurs, it will be identified by the system as a bad block. Like an electromechanical HDD, if this bad block is discovered during a write operation, the controller should replace it with one of the spares, automatically correcting the problem. Unfortunately, if the failure occurs after data has been written to it, the controller will likely make no effort to correct it unless the file containing the bad block is itself modified.

When a bad block of this type exists on an SSD, the results as far as the system goes, will be identical to a bad sector on an HDD. The system will attempt to continually read the file containing the bad block, it will fail every time, and the system will finally issue an I/O error. Typically the operating system or the application will issue some type of message indicating the operation did not succeed, and with a Mac you may witness the "spinning beach balls" as the system locks up and attempts to read the bad block.

Note that this phenomena is a rarity, and if you've recently purchased an SSD and this is a recurring problem, it's advisable to ensure the appropriate drive management software has been installed and that the drive's firmware is up to date. A new SSD experiencing excessive bad blocks is an indication that the memory used in the SSD is likely defective, the support software isn't installed, or the controller is not functioning properly.


Before attempting resolution, the scope of the problem needs to be determined. On HDDs, an excessive number of bad or weak sectors may warrant drive replacement. On SSDs if the number of bad blocks is increasing and the drive has been in use a long time, the drive is probably entering its end of life stages. Both of these are more or less extreme cases. SSDs may vary considerably by model and brand.

On an HDD, if tests on the drive indicate minimal damage it may be possible to reformat the drive by writing zeros to the entire surface of the drive or volume, which will force the controller to re-map all the bad sectors out. Step-by-step procedures for performing this can be found in our eBook Hard Drive Troubleshooting which can be obtained in the Downloads section of our web site.Some drive manufacturers also have tools that can be downloaded which will reformat and zero the drive, unfortunately many of the are only in Windows or MS-DOS formats.

If an SSD has  bad blocks present in existing data files, it's typically necessary to re-initialize the drive. Bad blocks encountered by an SSD during a write operation should automatically be mapped out, but those that went bad after being written will likely remain intact because most SSDs will not delete existing data files on their own. Many SSDs will come with their own application for performing this task and some won't. If such an application isn't available for your system, Disk Utility can be used to re-partition and re-format the drive, but needless to say, all data on the SSD will be lost. SSDs and their support applications and firmware will vary considerably from manufacturer to manufacturer, so the user will likely need to do some research to find out the specific procedures needed to correct these problems. 

Periodic delays when using the system.

The most common cause of periodic delays related to a drive will be bad or weak sectors on a mechanical hard drive and bad blocks on an SSD. These were covered previously, so in this section we'll look at other causes.

An HDD in the initial stages of failure may be periodically seizing or locking up. Typically this is accompanied by noise, but not always. An SSD doesn't have any electromechanical components, so this isn't a possibility.  During a surface scan test of the HDD, if the drive seizes or locks up, the scanning tool should detect it.

Other types of periodic delays can be caused by hardware malfunctions, software problems, and user problems. In most cases, hardware problems will be something fairly mundane, but never the less, not necessarily easy to identify. Software problems can be caused by applications as well as bugs in the operating system. User problems typically occur when the user is unaware that something they're doing is resulting in the problem or they're not familiar with the behavior of one or more system components.


For hard disk drives (HDDs) if the drive is having this type of problem the drive will need to be replaced. SSDs cannot have electromechanical component failure. The HDD will need to be thoroughly tested to ensure this is the problem. If the SSD or HDD cannot be confirmed to be the problem we strongly recommend reviewing the sections of this document at the top of this page with the links to System Problems, User Problems, and Software Problems for further information as this may not be a drive problem.

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Failure to successfully read or write data to the drive

Failure to read or write data to a drive can exist with respect to an entire drive, be limited to specific files on a drive, or be erratic in nature. This subsection applies to drives that can still be seen by the system. We've never encountered a condition where a user could write to a file system but not read from it unless there's a bad cable or other hardware fault or the permission settings of files are completely obscure and abnormal.

If the problem occurs with respect to an entire drive, the following common possibilities are:

Every drive, whether an  SSD or a mechanical hard drive, uses an index to find the location of files on the storage media. The index, which is usually a reserved area at the start of the drive, stores the locations of the blocks and sectors that make up all the files stored on a drive. When a user summons up a file for use, such as opening a document for editing, the operating system will interrogate the index to find the location of all the blocks on the storage media, fetch them, and then load them into memory for use.

If index corruption of the file system has occurred to such a severe degree that the OS cannot make sense of the file system the effected drives or volumes may be rendered read-only, and in extreme cases, may not be readable at all. This can often, but not always be corrected using the "repair" mode of Disk Utility.  Causes of index corruption can be attributed to bad sectors or blocks, faulty cables, the user isn't allowing the system to shut down properly, or a sudden power outage or surge. This type of problem is more prevalent in earlier versions of MacOS X (10.3 and earlier.)

We recommend performing a surface scan test on the first few gigabytes of the primary drive (not the volume) to determine if bad blocks or sectors are present. If bad blocks or sectors are present, see the section above titled "Delays from Bad or Weak Sectors". If the drive shows no signs of having bad or weak sectors or blocks, Disk Utility can be used to try to correct the problem, which can be done in recovery mode or by booting from an external drive with a full installation of the operating system. If this fails, you may wish to re-format the drive and re-install from backup.

If an SSD is entering its final life stages the drive may end up being marked "read-only" because it can no longer replace write cycle depleted cells with spares. How a specific SSD handles blocks during their final stages of life will vary considerably from manufacturer to manufacturer. Most SSDs should last many years before this happens. It's more likely that the index is corrupt or there's a permissions problem with the system.

Replace the SSD if it's confirmed that it's in its final stages of life.

If an SSD has been subjected to a transient or a sudden power outage, it may be suffering from SSD "sudden death" syndrome. Depending on the cause of the problem, this may actually completely destroy the SSD and all its contents, or the SSD may, almost miraculously, come back to life after sitting in an idle state for a while. We anticipate this being more of a problem on desktop units since laptops have batteries that will prevent complete power loss. Transients getting into a system can destroy anything, not just SSDs. If an SSD experiences this, it may look to the user like a new SSD was installed (unformatted, without data) or if the damage is severe enough, it may not be visible to the system.

Visit the manufacturers site for the SSD and see if they have any instructions or procedures that my revive the unit, if possible. We would also recommend performing a web search to find more information about the specific model and if it's susceptible to this problem.

If there is little or no space left on the drive, whether it's an SSD or a mechanical drive, it will become difficult, and in some cases impossible to write data to the drive. If no free space exists, you simply cannot write to that drive any longer, but such a drive should be readable, often with considerable delay.

The solution is obvious: delete data you no longer need.

Bad permission settings on a drive may cause failed read or write operations.  If this is the case, it may not be a drive or system problem, but rather the way the operating system works. Mac OS X is a Unix based system, and Unix based systems use varying permission levels for the type of user using the system. Not all users have access to all files on a system. To understand this limitation, it's best to review the various resources available regarding OS X for further details. It's also possible that an application may set permissions in such a way that the typical user is prevented from accessing them.

Use Disk Utility to repair the permissions on the drive. If the user is prohibited from accessing files or folders because they aren't authorized to do so, they should adjust their permission settings so they can be allowed to do so.

The file system frequently needs to be "repaired" by Disk Utility

If the file system frequently needs to be repaired by Disk Utility, it's most often caused by a user failing to shut the system down properly or ejecting drives incorrectly. However, if the system and drives are being treated properly, it usually implies that files in the system are "disappearing" because of the presence of developing bad sectors in a hard drive, bad blocks on an SSD, or there's a cable problem. These are described in the preceding sections regarding problems with bad/weak sectors on a mechanical hard drive, bad blocks on an SSD, and periodic delays caused (typically) by failing cables or connectors.


See the section above titled Delays From Bad and Weak Sectors above, and test the drive to see if there are problems. If the HDD or SSD is functioning properly, we recommend reading the article associated with the link System Problems at the top of this page.

Running applications lock up for no apparent reason

When running applications lock up for no apparent reason, the chances of it being drive related, software related, operating system related, hardware related, or user related, are generally about equal. There is no such thing as  a "perfect" operating system - all operating systems have bugs, as do all applications. Users may likewise be unaware of the fact that they're taxing the system CPU, memory, or even available drive space. A drive or its supporting components should really only be considered a suspect after the others have been eliminated as potential problem makers. Problems with the logic board or some of the components it supports, such as the Airport card may also cause these problems.

Read the section above titled Delays From Bad and Weak Sectors and perform a surface scan on the drive to see if bad or weak sectors are present. If the drive is functioning properly, review the articles associated with the links titled User Problems, System Problems, and Software Problems at the top of this page.

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The drive is no longer recognized by the system

This section will deal with a drive that is not recognized by the system at all. For problems regarding a drive that won't boot but is visible to the system, see the next section. Both of these symptoms have similarities and it may be advisable to read both before taking any action.

A drive that is no longer recognized by the system may have the following problems:

It's rarely possible for this type of problem to be a user induced problem, unless they've ejected the drive improperly, or in the cases of external drives, forgot to turn them on and/or plug them into the system (it happens!!)

A drive failure, whether SSD or mechanical hard drive, can be caused by a controller card failure, which will make the drive invisible to the system. Such a drive will not show up on any system, regardless of its platform. For mechanical drives, if this has occurred, it's possible the drive may still spin up and appear to turn on, but fail to communicate with the system. In some cases the supply section of the controller card may fail and not provide adequate power to the drive itself, making the drive appear like it's not even connected to the system. Additionally, if the electromechanical components on an HDD fail, the drive will often not register with the system properly.


Replace the HDD or SSD.

A failure in the system components supporting the drive is often a logic board or system problem.


We recommend reviewing the article associated with the link titled System Problems at the top of this page. The problem can be a failure in the logic board, a problem with the supply cable to the drive, or a supply problem.

An incompatible interface or outdated drivers typically won't be a problem with electromechanical hard drives, but it may be a problem with some SSDs. If an SSD isn't recognized by the system, it may be incompatible with it.

Check the manufacturer for compatibility information, firmware updates, and possible drivers that may be needed for the system.

If the OS can't understand the drive format it may very well be recognizing the drive, but just not displaying it to the user in Finder or the desktop version of Disk Utility. This can occur if the drive isn't formatted, it's formatted using a file system type that MacOS doesn't understand, or it's under the control of special drivers. It's also possible that the drive has been erased or reformatted without the user knowing about it.

We recommend becoming familiar with the command line version of Disk Utility, which is named diskutil. The "list" option of diskutil may very well make it clear that the drive is actually alive and recognized by the system, it simply isn't included in the end users display of viable drives because the format isn't recognized and/or acknowledged by MacOS. This problem is most common on external drives.

NOTE: Not all versions and formats of OS X are compatible with one another. Although the latest versions of OS X can typically deal with volumes from earlier releases, the opposite is not necessarily true. Core Storage, which was introduced in MacOS 10.7 (Lion) is not understood by versions of OS X earlier than this, and Lion based systems will not understand what a Fusion Drive is (Fusion Drives became available in Mountain Lion, or MacOS 10.8). Because of file system format changes, a user booted on a Mountain Lion volume may attempt to read data from a MacOS 10.10 (Yosemite) and newer volume only to find it reporting I/O errors because the resource forks in Yosemite aren't understood by OS X versions earlier than Mavericks (MacOS 10.9). These aren't the types of I/O errors generated from a bad hard drive or faulty I/O cable, but they may be shown as such to any applications trying to access them. If a user attempts to boot from a drive that's formatted for a later release of OS X than the system can support, they may be greeted with a prohibited sign indicating the OS on that volume is not compatible with the system they are using.

If the drive is an SSD and it's been subjected to conditions capable of causing SSD "sudden death" syndrome, the drive may be damaged beyond use and no longer seen by the system.

If SSD sudden death is suspected, we recommend contacting the vendor to try to find out if the drive is salvageable.

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A boot drive will no longer boot

This section will deal with a drive that is recognized by the system but won't boot. For problems regarding a drive that isn't visible to the system, see the preceding section.

A drive that will no longer boot may have the following problems:

Damaged boot regions occur when bad sectors or blocks develop in the boot region of the drive. If this type of damage occurs, the drive almost certainly won't be seen by the system. If the command line version of Disk Utility  named diskutil is used to list the drives, such a drive may show up as a single entity, such as "disk0" with no accompanying volumes or support partitions. Severe damage to the index files may have a similar effect. 


The first few gigabytes of the primary drive will need to be scanned using a scanning tool and if there are bad sectors or bad blocks found the drive will need to be treated as was described in the section above titled Delays from bad or weak sectors. If damage is severe the drive will need to be replaced. An HDD will require bad blocks to be mapped out if possible, and an SSD will likely need to be re-initialized.

If a system configuration setting becomes corrupt, the system may fail to boot. When the operating system boots, the first thing it tries to do is look for hard drives so it can load the operating system kernel. Information about the drive to use at start up is stored in the volatile memory on the logic board. If this drive information is corrupt, you may be greeted with a white screen, and it may eventually show a prohibited (circle with a diagonal line through it) sign, a flashing question mark, or simply stay in a locked up position with the white screen. Some of these symbols are shown in the diagram below.

        Failure Symbols

The System Management Controller, NVRAM, and PRAM all store parameters that the system can use to store system settings including boot information. These can become corrupt, and if they do, a reset may make a once invisible drive visible again. The following two links identify how to reset these parameters:  An NVRAM reset, with NVRAM containing startup drive selection  An SMC reset - The System Management Controller may also effect some boot operations
Before doing anything else, these resets should be attempted because they're easily done and frequently work. If the drive suddenly appears after the resets are done, it's usually ready to go.

If there are operating system bugs, the drive is probably an SSD or an external drive. Few, if any HDDs will exhibit this type of problem unless there is something highly unusual about the drive. Some SSDs that require drivers may have compatibility problems with the OS or expose bugs within the operating system. External drives will be described below in the section on external drives.


This will require contacting the manufacturer and checking compatibility of the drive with the system, and finding out if others are experiencing similar problems. Genuine operating system bugs with internal drives, including most SSDs is highly unusual.

If an incompatible interface or outdated drivers is present, make sure the the drivers are up to date and compatible with the system being used.  Interface incompatibility is virtually unheard of in contemporary systems but may be present with old systems or external drives. Problems with drivers of some sort will likely not exist in HDDs but may be present with SSDs.

With the release of Yosemite (MacOS 10.10) a new security feature has been added that requires some kernel extensions to be signed. If the extension, which is essentially a driver doesn't pass the security signing check, the system will likely fail to boot. Some TRIM software for third party SSDs available on the market suffers from this problem. 

As stated, interface incompatibility problems on contemporary drives and systems is virtually unheard of for internal drives, and dealing with old systems that may suffer from this type of problem should be considered beyond the scope of this document. Problems with kernel extensions or drivers should be handled by contacting the manufacturer of the product in question.

If the OS can't understand the drive format the drive is using, the drive may or may not be visible to the system (see the preceding section) but it almost certainly won't boot. Such a drive will typically show up using the command line version of Disk Utility named diskutil, which should tell you if the drive is present, but it may not show up in the desktop version of Disk Utility or in Finder.  If the drive is internal this type of problem was most likely created by improperly formatting the drive, or formatting the drive on another system and attempting to copy, clone, or install an OS image onto it without checking the drive format. This is actually not a drive problem but an installation or user problem.

Find out how this happened and correct the problem. Reformatting the drive will likely be needed. See the following note as well taken from the preceding section.

NOTE: Not all versions and formats of OS X are compatible with one another. Although the latest versions of OS X can typically deal with volumes from earlier releases, the opposite is not necessarily true. Core Storage, which was introduced in MacOS 10.7 (Lion) is not understood by versions of OS X earlier than this, and Lion based systems will not understand what a Fusion Drive is (Fusion Drives became available in Mountain Lion, or MacOS 10.8). Because of file system format changes, a user booted on a Mountain Lion volume may attempt to read data from a MacOS 10.10 (Yosemite) and newer volume only to find it reporting I/O errors because the resource forks in Yosemite aren't understood by OS X versions earlier than Mavericks (MacOS 10.9). These aren't the types of I/O errors generated from a bad hard drive or faulty I/O cable, but they may be shown as such to any applications trying to access them. If a user attempts to boot from a drive that's formatted for a later release of OS X than the system can support, they may be greeted with a prohibited sign indicating the OS on that volume is not compatible with the system they are using.

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A drive ejects itself for no apparent reason or won't mount

If a drive ejects itself for no apparent reason, it may be a bug with the operating system, but can also, particularly on external drives, be caused by a bad cable that's making the drive disconnect temporarily. Problems with external drives are covered below in the section dedicated to them. If an internal boot drive won't mount, the system won't start and the appropriate sections sections above should be consulted. If an internal boot drive ejects itself while in use, it's likely a problem with the drive itself or a problem with the system, particularly the logic board supply or cables supplying power to the drive.

In some cases if the system goes to sleep, the OS may eject the drive and on wake up the drive is seen by the system as disconnected. This problem typically occurs on external drives but it may occur on some internal drives as well. This problem is, in our opinion, an OS bug and can effect both SSDs and HDDs. As stated before, it typically effects external drives. Bugs of this nature seemed to appear on MacOS 10.8 (Mountain Lion) and later, and they will not occur with all drives or systems as they appear to be somewhat hardware dependent. In some cases this problem can be alleviated by opening System Preferences, selecting the Energy Saver option, and de-select (uncheck) the option to put the hard disk to sleep when possible. On laptops this should be done for both battery power and power adapter options.

If the drive is an electromechanical drive (HDD) it's possible that the drive may be entering the initial stages of electromechanical failure. If this is happening the drive may just appear to lock up or drop from site while in use. This will likely happen randomly. A surface scan of the drive may reveal the problem because the drive may seize while in use. Seizing will not occur on SSDs. If this is occurring the HDD will need to be replaced.

This type of problem can also be related to system problems associated with the logic board, supply, or supply cables powering the drive. Please review the section associated with the link System Problems at the top of this page for more details.

A drive appears to shut itself off for no apparent reason

If a drive shuts itself off for no apparent reason, the drive will be identified as being ejected (as in the preceding section), however in the preceding section the drive was still active. In this case, we're describing a problem where the drive literally powers down on it's own for what seems to be no apparent reason. Problems of this nature are typically caused by a problem with the hard drive's controller, the drive's on board power management circuitry, or the supply to the drive. In some cases this may be an artifact of drivers associated with external hard drives. In most cases this usually indicates a hardware problem, meaning either the drive itself is failing or the supply to it is failing.

An effort will need to be made to determine whether the problem is hardware related or not. If the drive is an HDD or SSD, the possibility of the problem being system related is high. If the drive is an HDD, it's possible the drive is entering the initial stages of electromechanical failure or the drive itself has a power supply problem. Both logic boards and some drive circuitry  often use fail-safe components to cut power in the event of a detected short circuit. Please review the section associated with the link System Problems at the top of this page for more details.

External Drive Problems

The external hard drive adds a few levels of complexity over internal drives. An external hard drive housing  will have an interface card inside it that will convert the hard drive data between that native to the drive and the type of interface being used (USB, Firewire, Thunderbolt, etc.).  Additional cabling and connections are used both inside the external drive housing as well as the I/O cables used to connect the external drive to the computer. An external hard drive will either have an external power supply or derive its power from the I/O port connected to the the external drive. Many external drives also use drivers that may or may not be the sources of problems.

Important Note: If an external drive appears to have failed completely, do not assume the drive itself inside the enclosure is dead. It's entirely possible the I/O card or supply regulation circuitry inside the enclosure might have failed. The only way to confirm this is to extract the hard drive from it's enclosure and evaluate it. If an external hard drive has confirmed bad or weak sectors  that doesn't necessarily mean the drive can't be read and data cannot be recovered. Since many hard drives may contain data like credit card numbers, be careful!

Any of the problems described in the preceding sections can apply to an external drive, whether it's using an HDD or an SSD as it's storage medium. The following sections will address only the problems specific and unique to external hard drives. If external hard drive problems exist, the preceding sections as well as the following sections will need to be reviewed.

If the external drive has problems that can't be directly correlated to the HDD or SSD inside the external enclosure, then the system will usually be exhibiting one or more of the following problems, which may be similar to problems associated with a standalone internal hard drive:

  1. Periodic delays that weren't present before or have become increasingly more common.
  2. Failure to successfully read or write data to the drive
  3. The file system frequently needs to be "repaired" by Disk Utility
  4. Running applications lock up for no apparent reason
  5. The drive is no longer recognized by the system
  6. If used as a boot drive it will no longer boot
  7. A drive ejects itself for no apparent reason
  8. A drive appears to shut itself off for no apparent reason
  9. Very slow system performance
  10. Data being written and/or read from the external unit is corrupt

If the problems are truly related to the  cables, connectors, or logic board traces running between the logic board I/O controller and the external drive connector or any of the cables or connections used both internally and externally to the external hard drive housing, they will have one or more of the symptoms identified in items 1, 2, 3, 4, 6, 9, and 10. If this is the case, see the section below titled Problems with the External Hard Drive, Support Circuitry, or the Logic Board.

If an external drive is no longer recognized by the system or its been configured as a boot drive and the system won't boot from it as described above in items 5 and 6, see the section below titled External Drive Won't Boot or Isn't Visible to the System.

If the drive ejects itself or appears to shut itself off for no apparent reason as described in items 7 and 8, see the section titled External Drive Ejects or Shuts Off While in Use.

If you're experiencing slow system performance as described in item 9, you should check the sections below titled External Hard Drive Performance Problems and possibly Problems with the External Hard Drive, Support Circuitry, or the Logic Board below. If the problem can't be correlated to the drive or the system, review the other sections of this site titled User Problems and Software Problems, which are linked at the top of this page.

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Problems with the External Hard Drive, Support Circuitry, or the Logic Board

Problems associated with an external hard drive, its support circuitry, or the logic board may all have similar symptoms. Problems associated with the heads and platters on an HDD or bad blocks on an SSD inside the external enclosure will typically be caused by bad or weak sectors/blocks on the drive. Problems of this nature will be repeatable during surface scan tests, hence obvious. Problems associated with faulty cables, connectors, external hard drive interface cards, or logic board traces will often be intermittent in nature and non-repeatable.

If intermittent problems are detected during a surface scan, the following are the likely culprits:

  1. Faulty logic board traces in the data path between the I/O controller and the actual I/O connector.
  2. Faulty cables inside the system that may connect an I/O connector board to the logic board.
  3. Faulty I/O cable (USB, Firewire, Thunderbolt, etc.) or connectors
  4. Faulty connections and cables (if applicable) inside the external drive enclosure
  5. In very rare cases, intermittent connections in the drive itself

Intermittent problems may generate delays and, depending on the severity, I/O errors as well. If using a diagnostic tool to perform tests on a unit, make sure the tool is capable of distinguishing between the two. Some test tools will detect an I/O error and flag it as a bad block or sector when it's actually caused by a timeout caused by intermittency. Intermittent problems can be difficult to detect because of their erratic nature, hence a tool capable of evaluating the interface ports is preferred.


In some cases, simple common sense procedures can isolate the problem. These include:

  1. Replace the drive's I/O cable with another as see if the problems disappear, thus identifying the cable as the problem.
  2. Verify that the cable's connectors are in good shape. Some connectors can "loop back" on themselves and in extreme cases, sections of the connector contacts may break or crack.
  3. Test the external drive on another unit to see if the problems continue
  4. If possible, plug the drive into a different port and see if the problems go away
  5. If using a port powered device that comes with an auxilliary plug-in supply, use it to see if the problems go away

If replacinig the cable with another cable fixes the problem, the problem was likely a bad connector or intermittent break in the original I/O cable.

If checking the connectors on both the computer and the cable reveal damage, that's the most likely cause. If the damage is on the port going into the computer, that port will no longer be usable unless replaced. Most connectors that connect directly into the logic board of a laptop computer and some desktop units cannot be easily repaired and the port should be seen as unusable.

If the extermal drive exhibits the exact same problems on another computer, replace the cable. If the problems persist, the problem is with something inside the external drive enclosure or with the external drive's connection to the cable.

If the drive will function properly on one port but not another, there's likely a fault in the path between the logic board's I/O controller and the connector leading to the I/O port. Unless the port is on an auxilliary board found on some desktop units, it implies that the port will probably be rendered unusable. This is likely due to a cracked circuit board trace, bad solder joint,  or damage from a transient.

If the external drive is connected to another unit and seems to work fine, but it will not work on any I/O ports using the original computer, there's likely a fault of some sort on the logic board, most likely in something directly or indirectly related to the I/O controller.

If a port powered device is in use, ensure that the system can genuinely provide adequate power to the drive. As drive loads change an inadequately powered device may behave intermittently.

External Drive Won't Boot (if configured as a bootable drive) or Isn't Visible to the System

If an external drive won't boot (if configured as a bootable drive) or it's no longer visible to the system, the following possibilities exist:

  1. The media on the HDD or SSD inside the enclosure is corrupt to the point the drive isn't seen as viable
  2. The controller on the drive has completely failed
  3. The drive isn't receiving power or adequate power
  4. The system settings have become corrupt
  5. The file system is extremely corrupt.
  6. The system doesn't recognize the filesystem or format of the drive
  7. Operating system bug
  8. Driver problems, if drivers are being used.

Before proceeding, we recommend reviewing the preceding section, particularly the Resolution section and attempting the same steps to help narrow down the source of problems. Whereas the preceding section dealt with problems that were primarily intermittent in nature, it's possible that an extreme condition may exist where the intermittent connection became a complete break. For example, delays and erratic behavior may be present if a cable has an intermittent connection in the data lines in the cable, but if there's a complete break in the data lines, no electrical contact will exist at all the drive won't be seen by the system. Using those procedures, you should be able to identify the following conditions:

Items 3, 7, and 8 from the numerical list above will be unique to external drives in this case, hence we will focus on them.

If the drive isn't receiving power or adequate power it may not be seen at all or it may appear erratically. Port powered drives are somewhat notorious for this. External drives with their own power supplies may have a failing supply.


If the drive is port powered and has an auxiliary supply, use it to see if the problems stop. If it doesn't have an auxiliary supply, attempt to power the unit via a powered hub that can guarantee adequate power. If the problems stop, then the port being used to supply power to the drive on the computer is inadequate. This type of problem will be most likely on laptop computers.

If the drive isn't port powered, the only thing that can be done is to try and power the drive by swapping the supply with a known good supply if possible. If the supply is internal to the external drive housing, the circuits related to the supply will need to be analyzed. In this case it might be easier to extract the drive from the housing and put it in another enclosure, but make sure the supply is the problem and the drive hasn't failed.

NOTE: Remember that if a supply or other component inside an external drive has failed, the drive inside the enclosure may still be fully functional.

If there's an operating system bug that prevents the drive from functioning with the system it first needs to be identified and confirmed. If confirmed (do a web search or visit the manufacturers web site) there may be workarounds available.

If the bug is confirmed as an OS bug, contact both Apple and the manufacturer to see if there's a work around.

Incompatibilities may exist between some implementations of a hardware interface and that implemented in the operating system. For example, in Mavericks we've had reports of USB drives that can't be seen by the system becoming "visible" if they're plugged into a USB hub, which in turn is plugged into the system. Essentially the USB hub is acting like a "Mavericks adapter." Whether this problem is cause by a problem with the operating system or the hardware is unknown. If possible, always check the external drive on other I/O ports (if available) or even other systems to ensure the drive is actually working.

Driver problems, if drivers are being used, may be incompatible with some OS verions. External drives often come with driver software and if it has been installed, check with the vendor and possibly do a web search to see if problems or conflicts exist with the operating system and the drivers and even the external hard drive in itself. Very often, when a new operating system version is released, some manufacturers may lag behind the release schedule, and incompatibilities may surface.


Contact the manufacturer of the drive for driver updates or considering using the drive with the drivers removed. Most external drives do not require drivers to work with OS X

External Drive Ejects or Shuts Off While in Use

If the external drive ejects or shuts off while in use, it may be caused by the following:

If drivers are turning off or disabling the drive after a period of inactivity it may be an energy saving feature of some drives. Some vendor supplied drivers have management software of their own which will put a drive to sleep over a period of inactivity. Sometimes the operating system may see these not as sleep events, but rather ejection events and report them accordingly.


Contact the vendor of the drive to ensure the drivers are compatible with the system. It may be necessary to disable all sleep options both on the driver and OS side to disable drive sleep. It may be a good idea to attempt using the external drive with the drivers uninstalled.

Firmware bugs may cause a drive to fall asleep, power off, or appear to eject itself for no apparent reason.

The web site for the manufacturer should be consulted to find out if such problems exist. If needed and possible, update the drive's firmware.

Operating system bugs and conflicts with some types of hardware  may cause a drive to appear to eject when entering sleep mode. Our experience indicates that in most of these cases, when the system goes into sleep mode some versions of OS X (usually 10.8 or later) either interpret the drive as being ejected or it actually ejects it. This differs from a driver related bug (described above) because this will occur without any drivers installed.

We recommend visiting the manufacturers website for information about this problem. If you're convinced it's an OS bug, file a bug report with Apple.

If the unit has bad I/O cables, a faulty power supply or source, or the drive in the external enclosure has a fault please refer to the preceding sections. The identification and resolution will be identical

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External Hard Drive Performance Problems

A few years ago, one could purchase an external hard drive intended to be used on Windows, re-format it in an HFS format, and put the drive to work. Unfortunately, some of the more recent external hard drives on  the market may exhibit some of the following "features," if you want to call them that:

Many external drive manufacturers seem to producing some units that deliberately trade off performance for low power consumption. If you're experiencing what appears to be seemingly lower performance with an external drive, it might be one of these models.

Some drives will not allow the user to deliberately or fully utilize all the controls of a drive unless the drive's own firmware and software is installed in the unit. This can be problematic if the operating system gets updated and the manufacturer doesn't provide updates for the software in a timely manner. If such a drive is put into use very often it may put itself to sleep without regard to operating system settings. When the user attempts to access the drive, the drive needs time to reactivate itself and spin up, which may take tens of seconds. Sometimes, if the drive is in this type of sleep mode, even attempting to save a file may end up taking tens of seconds.

Some drives are now also using proprietary controllers on the actual drive inside the external housing itself.  In some of the cases we've had reports on, the interface card/drive controller are hardware encrypting data on the drive. If a user attempts to extract the drive from the external housing and put it into use, it won't be seen as a "regular" hard drive by the operating system. How widespread this type of configuration is isn't known, but at this time it isn't common.

Since the power consumed by the drive is related to the drive speed, another way to save power is to reduce the spindle speed of the drive and vary it with data access. Data rate transfers can be directly correlated to the spindle speed, and if the drive is using a lower speed to save power, the data rate will be correspondingly lower. Under heavy use, these drives typically "ramp up" to full speed, and slow down when under light or no load. Under light or no load, this can effectively make the drive appear to be very slow at times. This typically applies to HDDs, not SSDs.

Excessive head parking and drive timeouts may also be used to cut power, although in many cases some individuals may consider these artifacts to be bugs in the firmware. Excessive head parking will slow down a drive and in some cases, sound similar to the clicking noises that sometimes appear when a drive is failing. Prolonged timeouts may also be present in some drives where the drive simply appears to stop working, usually for a fixed interval of time, such as ten or twenty seconds. This is an HDD only problem. SSDs have no heads so the problem will be non-existent.

If you suspect an external drive has any of these problems, we recommend visiting the manufacturers web site to see if there's any information about the suspected problem. Additionally, you might be able to find information on the web about other users having similar problems which might confirm that the problem is not unique to your unit. If the drive seems to be in working order, you may wish to visit the pages titled  User Problems and Software Problems for more possibilities.

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Scannerz, Scannerz Lite, FSE, FSE-Lite, Performance Probe 2, Phoenix, SpotOff, and Spot-O-Meter are Mac OS X universal binaries and support both 32 and 64 bit Intel based systems using Mac OS X versions 10.6 (Snow Leopard), 10.7 (Lion), 10.8 (Mountain Lion), 10.9 (Mavericks), 10.10 (Yosemite),  10.11 (El Capitan), and 10.12 (Sierra). Supported Intel based systems include all variants of the MacBook, MacBook Air, MacBook Pro, iMac, Mac Pro, and Mac Mini.