Sorry, close this one.
My mistake for not paying close enough attention to the CHKDSK results (normally they run in a DOS window and the window closes automatically). However, I ran the CHKDSK manually in DOS, to look at the results, and found that one drive had a USN journal entry. I ran the command to remove the USN journal and now my drives match each other on unused space. See link below:-
click here ran the fsutil command as described above and it only took a couple of seconds to complete. It may help somebody else in the future.