Backup a hard disk to another hard disk

I have a laptop with two OS (windows and archlinux) which I want backed up and, because the laptop has small storage, I have a 2tb external hard disk which I also want backed up. I have a second external hard disk with 4tb that is meant only to backup my laptop and my first external hard disk.
First question is if using restic to backup an external drive is wise given that I the mount points of the disk to be backed up may change. I guess it should be fine but would like some tips. Second question is if I need 3 restic repos (one for each of the 2 operating systems plus external hard disk) in separate folders in the 4tb backup disk or 1 restic repo to manage all these locations is enough.

Thank you

Edit: changed “hard disk” to “external hard disk”