Erase and reinstall OS X Mountain Lion

You can erase your Mac and then reinstall OS X using the built-in recovery disk. Before you erase and reinstall, back up your essential files. If you’re installing on a portable computer, make sure your power adapter is connected and plugged in.

Important: To reinstall OS X, you need to be connected to the Internet.

  1. Choose Apple menu > Restart. Once your Mac restarts (and the gray screen appears), hold down the Command (⌘) and R keys.
  2. If you’re not connected to the Internet, choose a network from the Wi-Fi menu (in the top-right corner of the screen).
  3. Select Disk Utility, and then click Continue.
  4. Select your startup disk from the list on the left, and then click the Erase tab.
  5. From the Format pop-up menu, choose Mac OS Extended (Journaled), enter a name for your disk, and then click Erase.
  6. After the disk is erased, choose Disk Utility > Quit Disk Utility.
  7. Select Reinstall OS X, click Continue, and then follow the instructions.