I used to have my FB set up like this on a RaspberryPi 2B:
- SD card as root file system
- external USB HDD as data location for various service and also as backup location
FB’s backup is using borg so the backup location on the USB drive was a simple borg archive. I never needed to restore anything(*), but if need be I could simply plug it into another computer and restore anything I need .
After a migration to new hardware (raspi 4b) I decided to also install the root & boot FS to the USB drive. SD cards do not have good failover characteristics and I’ve run into issues with them a few times over the years. So my new backup location is a separate partition on the (single) USB drive attached to my Pi.
However I am thinking about seting up a remote second backup, too.
(*) Correction: I actually used the restore feature when migrating to the new hardware recently. I had some issues with tt-rss, but aside from that, the restore worked fine. I moved the borg archive over to the new hard drive and used it as backup location in FB’s configuration - all backups showed up in the web UI as expected.