I convinced a friend to "share" data backup. He'd have some disks at his home where I can back up important data outside of my home (in case of fire or something like that), and vice versa.
The setup I envision: at my home I'd use my old server (I'll be running Debian or Ubuntu), and he'll likely just have an old Raspberry Pi of mine with a few external hard drives. What I was thinking was just to use rsync to sync nightly.
Realistically, he won't do any server (Raspberry Pi) maintenance, so I'd want to be able to administer his OS. But I'd also want it to be set up so I don't have access to his files either on my server, or on his. Being an "enthusiastic amateur" (at best), I'm not sure how to set it up so I can't access his files on an OS where I have root access? The only thing I could think of was if his data was encrypted, but then I guess rsync wouldn't work? And I'm guessing that it could be a hassle when he needed to add files on his end (for him, mostly photos/videos; I'll probably just install nextcloud for him).
So with my (admittedly very little) experience, I can't seem to think of a good way to do this. Any pointers would be appreciated.