Mac OS X 10.6: Checking a disk to see if it’s going to fail
You can check the health of many disks in Disk Utility. Many disks include Self-Monitoring, Analysis, and Reporting Technology (S.M.A.R.T) hardware, which lets a disk test itself for potential problems.
To check if your disk is about to fail:
Open Disk Utility, in the Utilities folder in the Applications folder.
Select the disk in the list at the left.
If you see a message saying the disk has a fatal hardware error, the disk has an irreparable hardware problem that can’t be fixed by Disk Utility or any other utility that repairs software problems. Back up as much of the data on the disk as possible, and then replace the disk. See an Apple Authorized Service Provider for more information.
If you don’t see a message saying the disk has a hardware error but you still have problems with the disk, you can try to repair it with Disk Utility.