When the computer cannot start up normally, you may need to use fsck for disk repair. In some situations, file system errors may prevent the computer from starting up to a normal state. This could occur after improper shutdown, forced restart, or power interruption.
The fsck utility is run from the command line. This means that you must type a text command at a prompt (#), rather than using the mouse pointer to open an application. Examples include Mac OS X’s Terminal application and single-user mode.
Start in single-user mode to reach the command line. Note: If necessary, perform a forced restart as described in the Emergency Troubleshooting Handbook that came with your computer. On desktop computers, this is generally achieved by pressing the reset/interrupt button, which is marked with a triangle. On portable computers, this is generally achieved by pressing the Command-Control-power keys. If a portable computer does not respond to this method, you may need to reset the Power Manager.
– At the command-line prompt, type: /sbin/fsck -fy
– Press Return.
The fsck utility will go through five “phases” then return information about the disk’s utilization and fragmentation. Once the check is finished, if no issue is found, you should see “** The volume (name of volume) appears to be OK.”
If fsck alters, repairs, or fixes anything, it will display the message:
***** FILE SYSTEM WAS MODIFIED *****
Important: If this message appears, repeat the fsck command until it no longer appears. It’s okay if you need to do several “passes” of fsck, because first-pass repairs may uncover additional issues.
When fsck reports that, “** The volume (name of volume) appears to be OK.”, type: reboot
Press Return. The computer should start up normally and allow you to log in.
The -y flag tells fsck that you want to answer “yes” to all questions about fixing, repairing, or salvaging information. This is the optimal approach, as answering “no” to any question causes fsck to stop. You cannot determine that all necessary repairs have been made until fsck completes and gives its final report. The -f flag forces fsck to check “clean” filesystems when preening.
This information excerpted from Apple’s “Using Disk Utility and fsck for file system maintenance in Mac OS X” document #106214. More info here.