OS X Mountain Lion Bible

Reinstalling or Recovering OS X

Should your Mac's startup disk get damaged or corrupted, you can reinstall OS X on it using several methods. Obviously, try repairing the disk first using Disk Utility, as Chapter 31 explains, because that could well solve the issue preventing you from starting up from that disk. (I suggest you make a bootable USB thumb drive, DVD, or external disk with the OS X Mountain Lion installer on it so you can use it to start up and run Disk Utility if there's ever an issue with your startup disk and its Recovery System.)

You also can use the Recovery System in OS X. To run the Recovery System, restart the Mac and hold +R while restarting until the OS X installation window appears. You also can get to it by holding Option during startup until the connected disks display; click Recovery HD or use the → or ← key to go to Recovery HD and then press Return. The Recovery System uses an invisible partition that OS X installed on your disk when you installed OS X. It also requires that you have an active Internet connection, so it can check for updates with Apple and verify your Mac is legitimate.

Note

If you can't run the Recovery System and don't make a copy of the OS X installer disk image on a bootable disk, you can sign in to the App Store from another Mac, re-download the installer from the Purchases pane, connect your Mac to that other Mac via Target Disk ...