Sounds like you may have file-system corruption.
Run a disk check ('chkdsk', or by using the GUI tool in the drive properties), as this will help determine if there's file-system corruption and/or possible drive failure.
If it reports problems on the drive, run it with the repair option (e.g.: chkdsk /f
) and after it finishes up try your defrag again (assuming chkdsk reported no fatal/uncorrectable errors).