1. Be sure the computer is shut down.
2. Press the power button.
3. Immediately after you hear the startup tone, press and hold the Shift key.
Tip: The Shift key should be held as soon as possible after the startup tone but not before.
4. Release the Shift key when you see the gray Apple and progress indicator (looks like a spinning gear).

To run OS X File System Check:

1. Click on Apple menu – Restart
2. During bootup, press and hold Command + S to startup in Single User mode
3. At the command prompt local host:/ root# / type the following:
/sbin/fsck –fy
4. Press the Return key. This executes the File System Check and allows any repairs needed. Notice the display indicating what is being checked (HFS Plus volume, Extents Overflow file, Catalog file, multi-linked files, Catalog hierarchy, volume bit map, volume information)
5. If errors are found, you’ll see the message FILE SYSTEM WAS MODIFIED.
Run the File System Check command again until all errors are repaired and you see the message The volume Macintosh HD appears to be OK
6. At the command prompt local host:/ root# / type the following:
reboot