2. Run fsck (file system check) from the command line (Boot, hold CMD+S after the chime. When prompted at the bottom of all that scary command line stuff, enter '/sbin/fsck -fy' then hit return).
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3. The ultra-cautious among us (though not overly cautious) will boot the machine from an installer CD and repair the disk (at least verify it) with Disk Utility.
These steps do the same thing.