If you want to sell your iMac or perform a clean installation of Mac OS X, you may need to completely erase the hard drive. Mac OS X uses a tool called Disk Utility that allows you to completely erase hard drives, USB flash memory sticks and rewritable CDs and DVDs. In addition, the Mac OS X Install DVD that came with your iMac lets you use Disk Utility from the DVD and completely erase your system hard drive.
Turn your iMac on and insert the installation DVD that came with it. Hold down the "C" key as your iMac boots to boot up with the DVD.
Choose your language on the installation wizard and press your keyboard's right-arrow key to continue. Select Utilities from the menu and select Disk Utility. Click your iMac's hard drive and select the Erase tab. Click Security Options if you want to use one of the advanced erasing techniques, such as writing zeros to the drive, and click "OK." Click "Erase" twice to erase your iMac's hard drive.
Click the Disk Utility menu on the left of the screen and select Quit Disk Utility after the process completes.
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