In very rare circumstances, the NTFS Metafiles $MFT or $BITMAP may become corrupted and result in lost disk space. This issue can be identified and fixed by running a chkdsk /F against the volume in question. Toward the end of chkdsk, you receive the following message if the $BITMAP needs to be adjusted:
Correcting errors in the master file table's (MFT) BITMAP attribute.
CHKDSK discovered free space marked as allocated in the volume bitmap.
Windows has made corrections to the file system.