Windows disk partition gap

Windows XP Disk Defragmenter report shows a constant gap in disk usage on a number of disk partitions on my system. I'm not referring to the little transitory gaps that occur. In disk D below, the gap in question is the one under the word "defragmentation". In disk P below, the gap is the one under "usage before def" the but a bigger one. The C partition doesn't have this anomaly. The size and placement pattern isn't obvious. It is as though there was an area, a no-man's land, that both the file system and the defragmenter avoid. These gaps survive daily use and defragmentation. I don't believe this is a residue from a paging file -- it should show up in green, anyway. Recycle bin is empty.

Any ideas?

Disk D (20 Gig):

Disk P (40 Gig):

That is probably the space reserved for the MFT, which will only be used for files if the disk gets really full. This empty space allows it to grow for a while without getting fragmented.

References:

• How NTFS reserves space for its Master File Table (MFT)