Normally, a hard drive breaks down over time due to any number of reasons. A hard drive is mechanical, so it will fail eventually. Clues can be a slowing down of your computer responses or sudden freezes, system crashes, start up problems, documents failing to open, unusually long application launch times, or unusual sounds like grinding or clicking noises. These warning signs mean that hard drive failure is imminent, so you better back up your data if you can still access your files.
If think you might be having hard drive problems, fortunately, Macs have a utility to check the hard drive. The best way to keep the hard drive in optimal condition is to check it periodically to ensure it is in good working order.
To check the hard drive on a Mac, go to Applications/Utilities or type Disk Utility into Spotlight. Open Disk Utility. Make sure Enable Journaling, located on the toolbar, is turned on. In case of a crash,  journaling will allow you to restore the drive. If the button is dimmed, it means journaling is already turned on.
Select the hard drive icon on the left. Self-Monitoring Analysis and Reporting Technology, S.M.A.R.T, is built into a Mac’s hard drive. S.M.A.R.T. is a drive self-test. S.M.A.R.T. monitors the condition of your hard drive and lets you know if it is “verified” or “failing”. If the status indicates anything but verified, now is the time to make a back up of your hard drive, because this status indicates the drive is failing.
With Disk Utility, you can either “Verify” or “Repair”. Choose Verify for a startup disk and Repair Disk for a non-startup disk. When you choose Verify, the utility just checks for problems and lets you know if the disk needs repair. To run Verify, click the First Aid tab and then click Verify Disk. If Disk Utility tells you the volume needs to be repaired, run the Repair Disk feature as directed in the next paragraph. Repair Disk can fix some types of catalog and data corruption, but it can’t correct an actual hardware malfunction. “Repair Disk Permissions” fixes faulty permissions. Sometimes when files won’t open, it can be because of incorrect permissions settings.
“Repair Disk” corrects problems with the drive directory. The directory keeps track of where files are stored. To run Repair Disk, boot from your OS X Install disc or another external backup of the OS. You have to boot from an another drive, because you cannot repair a disk you are booted from. Insert the install disc, press and hold the C key during start up. Once the Installer window appears, access Disk Utility via the installer’s Utilities menu.
If you don’t have the install disc, you can use safe mode at start up to repair the directory. To go to safe-mode, hold the shift key at startup until you see the words “Safe Boot” on the screen. In Safe Boot mode, Mac performs checks and repairs much the same as when you run the Repair Disk in Disk Utility. It also deletes the dynamic loader shared cache, a known cause of blue screens during startup, especially after a Software Update.