When troubleshooting file system problems, try these suggestions:
If you are having problems accessing files on a local hard drive, use Disk Utility's First Aid to verify and repair any problems with the disk drive.
If a computer becomes unbootable, a corrupted or deleted system file might be the cause. To fix these problems, try the following:
Put the computer in Target Disk Mode by pressing the T key while starting up the computer until the FireWire logo appears. In Target Disk Mode, the computer acts as a FireWire drive and can be connected to another computer, allowing you to run diagnostics, examine the drive contents, and recover and back up the files on the drive, even though the drive is unbootable. Now the computer's internal startup disk acts as an external FireWire drive that can be connected to another computer, allowing you to run diagnostics, examine the drive contents, and recover and back up the files on the drive, even though the drive is unbootable. You can also enter Target Disk Mode using Startup Disk preferences. Finally, you can "bless" the drive by using the bless command from the command-line interface (for instructions on its use, enter man bless).
Reinstall the operating system to replace the missing files. When the operating system version on the Mac OS X Install DVD is older than the one on the hard drive (as would be the case if you applied any system software updates), you'll need to use the Archive and Install option, which will enable you to install the older operating system without erasing the drive. You'll still need to apply updates to make the operating system version current.
