You could try with a USB stick to test, that’s what nostr:npub1uzt238htjzpq39dxmltlx60vxym9fetk9czz6kddq6fhvkf4z3usy9qtrh did and it worked. 😁
Start small and back up a simple service that doesn’t take up a lot of space (e.g. Vaultwarden) and go from there.
My guess is most external drives would work. I think you just have to make sure the filesystem is exFAT.
Good luck and keep us posted. 🤙