There are several methods of getting around this problem.
Booting from the Recovery HD partition or a different device: The Recovery HD is a special partition on your startup drive that has been present since OS X Lion and later. You can also boot from another drive that has a bootable system on it, or your OS X Install DVD, if you computer came with one. Safe Mode: This is a special booting method that forces your Mac to perform an automatic disk check and repair as it tries to start up. Single User Mode (fsck): This is another special startup method that allows you to run command line utilities, such as fsck, which can verify and repair hard drives.
Boot From Recovery Partition or Alternate Device
To boot from the Recovery HD, restart your Mac while holding down the command (cloverleaf) and r keys (command+r). Select Disk Utilities in the window that opens. You can also boot from another device, as long as it is bootable, including a bootable USB flash device, another hard drive, or an OS X Install DVD. To boot from another hard drive or a USB flash device, hold down the option key and start up your Mac. The Mac OS startup manager appears, allowing you to select the device to boot from. If your Mac came with a DVD install disc, insert the DVD into your Mac, and then restart your Mac while holding down the letter c key. Once your Mac finishes booting, use Disk Utility’s First Aid feature to verify and repair your hard drive.
Boot Using Safe Mode
To start up in Safe Mode, hold down the shift key and start your Mac. Safe Mode takes a while, so don’t be alarmed when you don’t see the desktop right away. While you’re waiting, the operating system is verifying the directory structure of your startup volume, and repairing it, if necessary. It also deletes some of the startup caches that may also be preventing your Mac from starting successfully. Once the desktop appears, you can access and run Disk Utility’s First Aid tool just as you normally would. When First Aid is finished, restart your Mac normally. Not all applications and OS features work when you boot into Safe Mode. You should use this startup mode only for troubleshooting and not for running day-to-day applications.
Boot Into Single User Mode
Start your Mac and immediately hold down the command key plus the letter s key (command+s). Your Mac will start up in an environment that looks like an old-fashioned command line interface (because that’s exactly what it is). At the command line prompt, type the following: /sbin/fsck –fy Press return or enter after you type the above line. Fsck will start and display status messages about your startup disk. When it finally finishes (this can take a while), you will see one of two messages. The first indicates that no problems were found. ** The volume xxxx appears to be OK. The second message indicates that problems were encountered and fsck attempted to correct the errors on your hard drive.
FILE SYSTEM WAS MODIFIED
If you see the second message, you should repeat the fsck command again. Continue to repeat the command until you see the “volume xxx appears to be OK” message. If you don’t see the volume OK message after five or more attempts, your hard drive has serious problems that it may not be able to recover from.
