When to Run First Aid in Mountain Lion's Disk Utility
The First Aid option in Mountain Lion's Disk Utility application is not often used. In most cases, after you’ve booted successfully from the Recovery HD or another bootable disk, the first logical troubleshooting step is to use the First Aid option in the Disk Utility application.
Every drive has several strangely named components such as B-trees, extent files, catalog files, and other creatively named invisible files. They’re all involved in managing the data on your drives. Disk Utility’s First Aid feature checks all those files and repairs the damaged ones.
One last thing: If you booted from a disk other than the Recovery HD partition, you’ll have to find and launch Disk Utility on that disk before you can follow these instructions:
Boot from the Recovery HD volume by restarting your Mac while pressing the Command and R keys.
The OS X Utilities window appears.
Select Disk Utility and click Continue (Recovery HD).
When the Disk Utility window appears, click the First Aid tab to select that function of Disk Utility.
Click the icon for your boot hard drive at the left of the Disk Utility window.
Your boot drive is the one with OS X and your Home folder on it; mine is called Mountain Lion HD.
Click the Repair Disk button.
Your Mac whirs and hums for a few minutes, and the results window tells you what’s going on. Ultimately, First Aid tells you (you hope) that the drive has been repaired and is now okay. If so, go back to work.
Quit Disk Utility by choosing Disk Utility→Quit Disk Utility or by pressing Command+Q.
Reboot without holding any keys down.
If First Aid finds damage that it can’t fix, a commercial disk-recovery tool, such as Alsoft’s DiskWarrior or Prosoft’s also-excellent Drive Genius, might be able to repair the damage. And even if First Aid gave you a clean bill of health, you might want to run DiskWarrior or another third-party utility anyway, just to have a second opinion.
Make sure you’re running a current version; older versions might not be compatible with OS X Mountain Lion boot disks.
If everything checks out with First Aid, restart and try to boot from your hard drive again. If you still get the prohibitory sign, you can try booting into Safe Mode.