Try running 'FSCK';
To run fsck, you first need to start up your Mac in single-user mode. Here's how:
1. Restart your Mac.
2. Immediately press and hold the Command and "S" keys.
You'll see a bunch of text begin scrolling on your screen. In a few more seconds, you'll see the Unix command line prompt (#).
You're now in single-user mode.
Now that you're at the # prompt, here's how to run fsck:
1. Type: "fsck -f" (that's fsck-space-minus-f).
2. Press Return.
The fsck utility will blast some text onto your screen. If there's damage to your disk, you'll see a message that says:
***** FILE SYSTEM WAS MODIFIED *****
If you see this message--and this is extremely important-- repeat running fsck. It is normal to have to run fsck more than once -- the first run's repairs often uncover additional problems..
When fsck finally reports that no problems were found, and the # prompt reappears:
3. Type: "reboot" to restart,
or type "exit" to start up without rebooting.
4. Press Return.
Your Mac should proceed to start up normally to the login window or the Finder.