The minute your computer's hard disk starts to act flaky, make sure you have an up-to-date backup. Then you can do some simple diagnostics and repairs. Both Windows and Mac OS come with built-in hard-disk utility software that scans your hard disk for errors and attempts to fix them.
STEP 1: Double-click on My Computer to open the My Computer window.
STEP 2: Select the disk that you want to diagnose and repair.
STEP 3: Choose Properties from the File menu. You should see the Properties window for the drive that you selected.
STEP 4: Choose the Tools tab.
STEP 5: Click the Check Now button under Error Checking Status.
STEP 6: Depending on your version of Windows, choose either "Thorough" or "Scan for and Attempt Recovery of Bad Sectors."
STEP 7: Click on Start.
For Mac OS
STEP 1: If you're running Mac OS 9.2 or an earlier version, search your hard drive for a file called Disk First Aid. You can use this Apple-supplied utility to verify and repair disk errors.
STEP 2: If Disk First Aid says it can verify but not repair your disk because it's the startup disk, you should restart your Mac from the System CD-ROM disc that came with your computer. Hold down the C key during startup to force the Mac to start from the CD-ROM. Older Macs, pre-G3, need to press the Command- Opt-Shift-Delete combination.
STEP 3: If Disk First Aid fails to fix all the problems it detected the first time you run it, try running it a few more times
STEP 4: If you're running Mac OS X, you'll find that Disk First Aid has been made part of the Disk Utility program, which you'll find in the Utilities folder in the Applications folder. Click on the First Aid tab after launching Disk Utility.