Disk Utility's First Aid Pane on Your MacBook
From the First Aid pane, you can use Disk Utility on your MacBook to verify (or check) any disk (well, almost any disk) for errors, as well as repair most errors that it finds. Here are the two exceptions when the buttons are disabled:
The start-up disk: Disk Utility can’t repair the start-up disk — that’s Mac talk for the boot drive that contains the Mac OS X Lion system you’re using at the moment — which makes sense if you think about it, because that drive is currently being used!
If you have multiple operating systems on multiple disks, you can boot from another Mac OS X installation on another drive to check your current start-up disk. Or you can boot your system from the Recovery HD partition and run Disk Utility from the menu.
Write-protected disks: Although you can use the Disk Utility to verify CDs and DVDs, Disk Utility can’t repair them — that’s because they’re read only. (Sound of palm slapping forehead.)
You usually can’t repair a disk that has open files that are currently being used. If you’re running an application from a drive or you’ve opened a document that’s stored on that drive, you probably can’t repair that drive.
You can also elect to verify and repair the permissions (or privileges) created on your disk by the Apple Installer during application installations. If you can’t save or move a file that you should be able to access, check that drive for permissions problems.
Although you can’t fix disk errors on a boot drive, you can verify and repair permissions on any volume that contains a Mac OS X installation (whether it was used to boot your MacBook or not).
In order to verify or repair, you must be logged in as an admin-level user.
To verify or repair a drive, first select the target volume/partition in the list at the left. To check the contents of the drive and display any errors, click the Verify Disk button. Or, to verify the contents of the drive and fix any problems, click the Repair Disk button.
Disk Utility displays any status or error messages in the scrolling list; if you have eagle eyes, you’ll note that the window can be resized so that you can expand it to display more messages. (You can also drag the dot between the left and right panes to expand the list.)
If your MacBook is caught by a power failure or Mac OS X locks up, however, it’s a good idea to immediately check disks after you restart your laptop. (Don’t forget that the start-up volume is automatically checked and repaired, if necessary.)