Repair A Macintosh Hard Drive

Steps

1. 
   1
   Have a backup handy just in case. There is a chance this can erase some data or just wipe out your entire hard drive. So, keep a backup handy.
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2. 
   2
   We are not going to reinstall Mac OS X.
3. 
   3
   Check if the disk needs repairs. You can check this by going into Disk Utility, clicking your disk in the sidebar, and clicking Verify Disk.
4. 
   4
   Save your work in all open applications.
5. 
   5
   Turn on the sound to a point that you can hear it. It will be helpful for booting into Safe Mode after you repair the disk.
6. 
   6
   Insert your install DVD. Don't do anything with it.
7. 
   7
   Restart your Mac (Apple Menu > Restart).
8. 
   8
   When you see the white screen, start holding down C. Don't let go until you see the usual Apple logo and spinning wheel.
9. 
   9
   Click the right pointing arrow in the dialog box that appears. Don't do anything else unless you want a new language.
10. 
    10
    Go to Utilities > Disk Utility. This disk utility is the only one that can repair disks.
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    11
    Select your disk in the sidebar and Click Repair Disk. Wait. Don't turn off the Mac.
12. 
    12
    Exit the installer.
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    13
    Quit Disk Utility and choose Utilities > Start-Up Disk.
14. 
    14
    Click the disk with Mac OS X and click Restart.
15. 
    15
    Hold the shift key as soon as you hear the start-up sound. Don't press it before the sound.
16. 
    16
    Keep holding it until a loading bar appears at the bottom of the screen. Booting will take much longer than usual, so just wait.
17. 
    17
    You are in Safe Mode. The sound doesn't work, you can't use the internet, and the dock lags.
18. 
    18
    Log in and launch Disk Utility.
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    19
    Click your disk in the sidebar and click Repair Disk Permissions.
20. 
    20
    Restart normally.
21. 
    21
    Restore any lost data and get back to what you were  doing.                                                                                                                                                            
    
       For safety, you should go to Applications > Utilities > Disk Utility, click your start-up disk in the sidebar and click Verify Disk to check it. However, when it says something is wrong, you have to repair it, or bad things will happen.
    (Note: These steps relate to Snow Leopard.)
    
    Source: WikiHow.com