Drives differ in size after sync

[nahydro]

Greetings:
New user here.

I have two 6T drives used as backup. Drive B is a mirror of Drive A, for redundancy.
After selecting and copying all files from Drive A to Drive B, the two drives differ in size by approx. 8 gig. I've repeated the operation several times, with the same results.

I installed FFS to do a comparison of the two drives and, if I am operating it properly, it finds only a single 400 byte .ini file that is not shared between the dirves. I am already aware of this single difference.

What could explain the difference between the two drive's volumes?

Thanks.

[nahydro]

Continued....(sorry, I couldn't find an option to edit my first post)

Also, I tried to set up my compare between A:/ and B:/, to compare the entire drives, however the compare process seemed to complete instantaneously. Would FFS compare all sub-directories under the root?

Thanks again.

[Zenju]

https://freefilesync.org/faq.php#explorer-mismatch