I've noticed recently that Drivepool deals with "Size" vs "Size on disk" differently inside and outside of the pool which causes drivepool to think the drive is bigger than it is.
From what I can tell:
When files are outside of the pool it uses the "Size on disk" i.e. "Used space" reported by Windows + "Free space" and calculates the size of the drive.
When files are in the pool it uses the "Size" not "size on disk" to calculate used space + "Free space" to calculate size of the drive.
This causes a discrepancy in the size it believes the drive to be and can also trigger file moves before required (e.g. move when 100gb is needs to be moved)
It seems like it's always done this, just caught my attention now since files were being moved when I knew it wasn't needed.
See below examples:
1: Folder outside of the pool. Notice size, size on disk and the drivepool measured size
2: Now I've moved that into the pool and notice what Drivepool (Top right) sees as the size. (Size + Free space)