OS X Lion: Reinstall Mac OS X
Under some circumstances, you may need to reinstall Mac OS X. You can reinstall Mac OS X while keeping your files and user settings intact.
- Choose Apple menu > Restart, and then hold down the Command (⌘) and R keys while the computer restarts.
- 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, and then click Continue.
- Follow the onscreen instructions. In the pane where you select a disk, select your current Mac OS X disk (in most cases, it is the only one available).
- To start the installation, click Install.
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: