In my last post I explained that the 38gb Properties box is not the correct one for the entire D drive, but only for one or more files on the drive - if you look at it's heading it shows Dr. J Work which is likely the file(s) it mistakenly represents as D. This is an error.
The correct Properties box for D is the other one which shows exactly what Disk Management shows and therefore confirms it.
I hope this makes it clearer.