I have a synology ds412+ nas.  It's a four bay unit with 2 x 2 TB and 2 x 3 TB hard drives, configured using the synology hybrid raid with 1 disk fault tolerance.

This should mean that there is around 7 TB of storage space - a bit less in practice.

Storage manager says that the capacity is 6.25 TB( ie a bit less than the 7 TB) with 5.71 TB used (and 556 GB used).  So far so good.

However, when I add up the disk space used, using windows properties of the various folders, total files space used only comes to 4.03 TB.  Question is where has the extra 1.7 TB gone.

Storage manager says that all disks are normal.  I have cloudstation activated and have cleared out the various recycle bins.

Has anyone got any ideas?

Thanks in advance