2 identical harddrives with unidentical used storage

[deekroepjee]

Hello Helpers,
I have 2 identical external harddrives I keep in sync with freefilesync (allways updated to latest version).
The drives are seagate 1TB, NTFS 1 partition.
I use windows 8.1 on a samsung system.

Freefilesync can, at the moment, not find any difference between the drives. However, windows notes that there is a difference is used storage space of 41Gb. Thus, one dive has 142GB available, the other only 101Gb.
The recycle bins are empty.
If I look in windows explorer, there is no difference in hidden files on the drives.

Could it be that Freefilesync produces hidden files on one drive that windows does not show? Maybe freefilesync puts the recycled files somewhere that I do not know of?

Any help is greatly appreciated.

Great program by the way!