I posted this in the bug reporting forum a couple of weeks ago and someone replied today that they were also seeing it. I am posting it here to see if it is more prevalent and if so, maybe it can get some attention.
I was defragmenting my external drive. The partition I was doing is 781gb and was not close to being full. After running overnight and not getting very far, I cancelled it this morning and figured I would move a couple of large files to another disk so it would have an easier job of it. When I tried to restart it as a quick defrag, it warned me that there was less than 10% free space on the disk, which seemed very strange since I did not get the warning the first time. I ran Treesize on the drive and it's only finding 358.5gb in files on the disk, but both it and Defraggler are claiming I only have 10.8gb in free space (and I should say that it was 13gb before I started and then cancelled a quick defrag.
CHKDSK and the internal disk check from Defraggler claim there are no errors on the disk. How do I get back my missing disk space???
