Carl wants to compare hard drives to see which is missing the files of the others. Leo says that kind of program is called a DIFF program. Kaleidoscope is one, and it's $69 with a free trial.
Another option is a DeDuplicator, which would enable him to copy all the files into a single hard drive from the others and then back them up. The Chatroom recommends Scooter Beyond Compare. WinMerge is free and open source.