Check Your Drives' S.M.A.R.T. Status
Most modern hard drives have built-in sensors and monitoring circuitry that form a system called S.M.A.R.T. (Self-Monitoring Analysis and Reporting Technology). The idea of S.M.A.R.T. is to detect the warning signs of potential problems before they occur. Although S.M.A.R.T. cannot detect every possible drive problem, it can provide one very valuable warning: "Your drive is about to have problems, so back it up and repair (or replace) it now!"
What does it work with? As of mid 2006, Disk Utility's S.M.A.R.T. indicator works with internal ATA and Serial ATA drives, but not with external (USB or FireWire) drives. Some external drives, however, have their own built-in S.M.A.R.T. indicators.
To check your drives' S.M.A.R.T. status, launch Disk Utility (find it in /Applications/Utilities) and select a drive in the list on the left. (Be sure to select the drive itself, not one of the indented volumes beneath it.) If the selected drive supports S.M.A.R.T., you should see this at the bottom of the window: "S.M.A.R.T. Status: Verified" (Figure 12). If you see "About to Fail" in red letters, back up the drive immediately. You can then use Disk Utility (or a third-party repair utility) to attempt to repair the drive, but more often than not, "About to Fail" indicates an imminent hardware failure that you cannot fix with software. Even if Disk Utility does appear to solve the problem, don't trust the drive with important data; replace it as soon as possible.
Figure 12. Disk Utility shows a drive's S.M.A.R.T. status.
To monitor your drives' S.M.A.R.T. status in the background (without having to remember to open Disk Utility), try SMARTReporter, which displays a status icon in your menu bar (http://homepage.mac.com/julianmayer/, free).