Mountain Lion's Recovery HD Partition
When you start your Mac, if OS X Mountain Lion displays a prohibitory sign, spinning-disc cursor, or kernel panic alert that doesn’t go away, the first thing to do is attempt to repair hidden damage to your hard drive with the Disk Utility program’s First Aid feature.
And to do that, you’ll have to boot from the Recovery HD partition. That’s because you can’t run Disk Utility’s First Aid feature on the current startup disk.
To start up from this magical disk (actually, a disk partition), here’s what to do:
Restart your Mac.
Press and hold Command+R until you see the Apple logo.
If your Mac doesn’t boot from the Recovery HD after Step 2, hold down the Option key while booting to display the built-in Startup Manager.
If you press Option after the startup chime instead of Command+R, the built-in Startup Manager appears. From this screen, you can click the Recovery HD icon, and then click the arrow below it or press Enter or Return to boot from it. Or, if you’re going to boot from a disk other than the Recovery HD, you can select it here.
Pressing Option during startup displays icons for all bootable volumes it sees and allows you to select one (including the Recovery HD partition).
Click the disk you want to start up from, and then click the arrow below it or press Return or Enter to start up your Mac from it.
This technique is quite useful if your usual startup disk is damaged or having an identity crisis during startup and the Command+R trick isn’t working to boot from the Recovery HD partition.
If you can boot from the Recovery HD partition (or a, external startup drive, clone, DVD-ROM, or other disk): If you see the OS X Utilities window after booting from the Recovery HD partition, hope flickers for your Mac. The fact that you can boot from another volume indicates that the problem lies in one of two places: your startup volume and/or the OS X installation on that volume.
Run First Aid in the Disk Utility application.
If you can’t boot from the Recovery HD partition, (or an external startup drive, clone, DVD-ROM, or other disk): If you can’t get your Mac to boot from the Recovery HD partition or another bootable disk, you might have to call for support or take your machine in for repairs.