If you expect your friend to accept having something plugged all the time, I assume you'd be ok with having a similar device at your place running their backup,

If you expect your friend to accept having something plugged all the time, I assume you'd be ok with having a similar device at your place running their backup, right ? In that case, you can buy a NAS for you, have them buy one for them, backup stuff on your own NAS and let the replication from your NAS to their NAS happen in the background. Bonus: backups/restore/browsing are much faster, and you can more easily accommodate multiple people wanting to backup at the same place -- instead of one device at your place for each of your friend, it's only one at your place to which all your friend's NAS backup to. I use Synology NAS, not with this setup, but I know they have the software to do exactly this.