Storage Pool full, but almost no files storable.

Hi,

I created a Storage Pool on my Server 2012 Standard with 4x2TB disks with parity redundancy and fixed provisioning.
After a few months the free space is about 47GB, so pretty full (5.45TB volume capacity).

However if I look at the files on this volume, it is only 2GB in size ...
So almost all of the disk space is somewhere in use, but I cannot see where.

Any idea?

Thanks