why does the drive space used vary between two disk drives holding identical files

Solution 1:

One thing that can happen (among others) is that NTFS keeps a journal which records things that happen to files and folders, such as move, copy, update. This is kept in a large, fairly well hidden file. I doubt that it or it's content are reported through chkdsk. There are APIs to read from it...but the file itself is not generally accessible. If one volume was an active volume for some period of time, and the other is a backup, then this can account for a lot of hidden size difference.

Also, I noticed that the reparse points are somewhat numerous on one and almost non-existent on the other. A naïve backup can effectively destroy reparse points...and can undo hard links (pointers to files in various folders that are really not copies but links to the same file...kinda like shortcuts). For example, most of the files in c:\Windows\WinSXS are hard links to other files in other locations on the same volume. When copying files off, a program should be track and recover the structure of the reparse points and hard links. Depending on the utility used, these files can be accounted for differently.