Using Defraggler 2.22.995, I noticed that it has issues dealing with NTFS “sparse” files.
– When analysing a 931GB partition which contains several complete or partial drive images, including one complete 931GB image file with an actual size of 206GB, and several partial 3TB image files with actual sizes up to 44GB, all created with ddrescue (from a Lubuntu live session) in “sparse” mode (-S switch), Defraggler doesn't list those files (all those with the “P” attribute), and seems to identify their clusters as “free space” (the whole partition is supposed to have 35GB of free space, yet the blue blocks take up about a third of the map's surface.
– When opening that same partition, Defraggler displayed a notification saying that I could release 16GB by emptying the recycle bin – it turned out that the recycle bin contained a bunch of ”sparse” files, purposely created as almost empty, also using a ddrescue -S command, with a total size of 16GB indeed, but an actual allocated size of only 13MB. Therefore that message is misleading, and again, Defraggler is confused by the “sparse” nature of those files.
EDIT : In this particular case, I used Defraggler specifically to determine where the clusters of that 931GB image file were located, which it usually does very well (as I wrote here or here or here). I didn't actually attempt to defragment the partition (which according to this thread would result in quite a bit of a mess).