On the disks in questions, do you have Shadow Copies enabled?
TO check, run "control system", and click on the "System Protection" link. Find the drives in question. If "Protection" is "On" for them, select the drive and hit "configure" and turn it off.
Also, if you "take ownership" of the "System Volume Information" folder, and delete it, that will also purge the shadow copies.
Also, you can use utilities such as WinDirStat to see the layout of all the files on the pool, what folder they're in and how much space they're taking up.
Also, if you right click on the the drive, and select "Properties" and let it sit for ... a while, it should show you the space used and the space used on disk (these are different numbers, and that difference is the "slack space" we talk about, and is attributed to "other").
If that equals what is on the disk, then you may just need to defragment the drive.
