Compare folders on two drives, copy remainder to third drive?

[Gkirk]

I have two hard drives I've copied onto a single share on an Unraid server, so there are about 2,000 folders on each drive backed up into a single directory of 4,000 folders.

Essentially I've got drives A and B merged to drive X. Now I've replaced drive A. So I need to compare drives B and X, and copy all the folders from X that aren't on B to drive A.

Seems like it'd be pretty easy to get a list of folders on X that aren't on B, and just drag and drop that list to A, but I don't seem to be getting anywhere at all.

Anyone have any ideas how to do this?

[Zenju]

Sounds like you'll need CTRL+T: https://freefilesync.org/manual.php?topic=tips-and-tricks#copy-to-alternate-folder