And are they drives, partitions on a single drive, or a combination?

What happens if you manually start a disk check for this disk? My Computer, right click the disk, select Properties > Tools and select "Check Now" under Error checking. This will schedule a disk check at the next startup.

I would carry out some housekeeping maintenance first, remove all temp files and TIF's, (Temporary Internet Files), do a manual anti-virus and anti-spyware scan, and backup all your 'can't afford to lose' personal files.

The chkdsk will take some time and it cannot be interrupted.