OS X Lion: Erase and reinstall Mac OS X
You can erase your computer using Disk Utility and then reinstall Mac OS X. 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.
- Choose Apple menu > Restart, and then hold down the Command (⌘) and R keys while the computer restarts.
- Select “Disk Utility,” and then click Continue.
- Select your startup disk from the list on the left, and then click the Erase tab.
- From the Format pop-up menu, select Mac OS Extended (Journaled), type a name for your disk, and then click Erase.
- After the disk has been erased, choose Disk Utility > Quit Disk Utility.
- If you’re not connected to the Internet, choose a network from the Wi-Fi menu in the right corner of the menu bar (in the top-right corner of the screen).
- Select Reinstall Mac OS X, click Continue, and then follow the instructions.
For more information about reinstalling Mac OS X, see this help topic:
Mac OS X Lion comes with a built-in recovery disk that you can use to reinstall Mac OS X, repair a disk, restore from a Time Machine backup, and more. To use the recovery disk, restart your computer while holding down the Command (⌘) and R keys. If you want, you can also create an external recovery disk. For more information, see this Apple Support article: