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:
Before doing anything else, these checks should be done because they're easily done and sometimes work.
If the resets fail to produce positive results, the first thing that needs to be determined is whether or not the drive is visible to the system at all. This usually requires booting from install media (if available), another bootable drive (usually external), or if you're a Scannerz user and you've created a Phoenix Boot Volume, then that can be used to boot the system. Once the system is up, Disk Utility should be brought up to see whether or not there's any indication of the drive being present.
If the drive shows up in Disk Utility, it indicates that the drive is visible to the system. It may show up with a volume name as a valid Mac volume, which typically implies either the drive is damaged from bad sectors or the index files are so corrupt that it can't be used to boot the system. If the drive is showing up not as a volume, but rather as a disk designation only (such as disk0s2) it may indicate that the drive is excessively damaged, or possibly has been reformatted with a foreign operating system Mac's aren't familiar with (this is usually rare but since Mac's can be put into Target Mode, it's possible.)
At this point it can't hurt to use Disk Utility to run checks on the drive, but keep in mind that it's not a surface scanning tool - it will only be trying to see if it can repair a bad drive index. If an index repair works using Disk Utility, you might be able to resume using the drive. If Disk Utility indicates there are no errors with the drive, it likely indicates that some of the critical boot files have either been deleted or lost in bad sectors. If Disk Utility fails its verification and/or repair of the drive, the drive index files may be too corrupt to be repaired and/or there may be bad sectors on the drive.
If the drive can't be dealt with properly using Disk Utility, if you're using a Phoenix Boot Volume or an external hard drive with Scannerz installed on it, you might wish to try and perform a surface scan on the drive, particularly the first few gigabytes, to see if it's registering any errors. Follow the steps described in the preceding section to accomplish this. If the drive registers surface scan errors, more than likely the drive needs to be repaired or replaced. This needs to be confirmed using Scannerz in Diagnostics Mode to ensure the problems are sector and not cable related. The first few gigabytes of a volume typically contain all the boot code and libraries needed to bring the system up. If bad sectors are detected and confirmed, see the section titled Dealing with Problems on the base page of this set of articles.
If both Disk Utility and Scannerz are reporting no problems, more than likely critical files in the operating system have been deleted. This is extremely unusual but can happen, particularly by accident.
If the drive can't be seen at all by Disk Utility, Scannerz, or the operating system, it usually indicates either a drive controller failure, a sudden catastrophic failure in the electro-mechanical components of the drive, or either a supply failure or a failure in the power cables supplying the drive with current.
In the event the drive controller has failed, unless you're interested in replacing the drive's controller card yourself, the drive should be considered dead. Replacing a drive controller isn't that difficult a job to do for someone with any experience working on computers, however it's critical that the controller replacement have the exact same type of card and ROM as the original. Manufacturers frequently update these, and a mismatch can end up causing problems. In many cases, however, the data on the drive can be recovered. A failed controller will yield a drive that does not show up at all in the system or when using Disk Utility.
Power sources and data cables can become disconnected or develop complete breaks in the wires inside the cables. This will obviously prevent the drive from ever starting up and being seen by the system. This will require opening the system up to investigate the problem.
Finally, logic board problems may exist that will render booting useless but allow the system to appear to be running (i.e. there is no system crash when booting) using an affected port. One way you might be able to check this is to try booting from another drive, install media, or a Phoenix Boot Volume. If all attempts to boot using auxiliary devices fail it's likely there's a problem with the I/O controller on the logic board or some of its supporting circuitry. If you can boot from an auxiliary drive, then the problem is likely not the logic board but possibly bad cable or supply connections feeding the drive. If the source of the problem is the logic board itself, the drive may very well be in good working order with all data retained.
Drive Ejects or Shuts Off While in Use
If the drive ejects or shuts off while in use, it's usually a supply problem or a cable problem but other possibilities exist as well. If the drive seems to be running and it's been ejected, it's more than likely there's an intermittent connection in the cable supplying power to the drive, but it can be logic board supply traces or intermittent connections in the supply lines of the logic board or power supply (if separate.) If it shuts off completely but can be restarted after shutting the system down and re-starting it, it's typically a problem with the supply or another component overheating in the system. This might be caused by faulty thermal sensors or actual thermal failure somewhere in the system. Correcting all of these problems will require opening the system up and troubleshooting the problem in more depth.
Hard Drive Performance Problems
Unless an internal hard drive, system, other components, or even software have been evaluated and found problem free, it's unusual for an internal hard drive to have genuine performance problems. However, there are some new, low power drives that may behave in an unusual manner when installed inside a system. Typically, these drives are intended to be placed inside an external enclosure for backup or basic storage use. They weren't designed to be high performance "boot" drives. The drives may exhibit excessive head parking, prolonged timeouts, or variable spindle speeds.
If you're experiencing problems and it's truly believed that the drive is to blame, our advice is to do a web search and obtain drive specifications and user reports on the drive to see if there's correlation between what's being observed with the system in use and other reports.