My Book Duo - Capacity difference

Hello all,

Hopefully picture is visible to all.

Can someone help me understand why I have such big differences between these mirrored drives. I own the My Book Duo 20TB (which is 2 x 10TB HD)

I use Beyond compare 4 to mirror the e: (as a back-up). Beyond compare states that there are no differences between the drives, but look at this picture. There are very big differences.

  1. Windows explorer shows 0.5TB difference
  2. There are a different number of files and folders
  3. Size are both different
  4. Size on disk are both different

Why are there so many inconsistencies in size ?