Deleted all files, confused about "Useage" report?

Hey all, I deleted all files from my My Book Live Duo and am trying to confirm I have “all” that storage space back. Below pic is the screengrab and it seems I am using 6gb out of 4tb. Not going to worry about 6gb, but in my head I think i should have no space used and cant find this 6gb of useage. Is that the WD software?

That space is used by the Operating System built into the MyBookLive.