OK, maybe it’s time to bring this back from the dead.
There’s apparently and unfortunately no elegant way to export and back a library up from RoonServer. It’s of course possible to do it, just a little bit inelegant.
For everyone with a fast line, having an option to do what the database does with Dropbox, but with BackBlaze or one of their competitors would be great. The real issue is other people.
For the server / docker users, allowing to add a mount, much in the same way that it’s possible to do so for the library itself, could do the trick. Let 'em deal with managing the rest, though maybe exclude the library from the choices (I can’t imagine someone, at some point, won’t pull that one 
).
For ROCK / Nucleus / the desktop users, something that’d work a bit like Apple’s TimeMachine, complete with incrementals, would probably be ideal. Prompt the user via the client interface to plug an external drive into a USB port every n days / weeks / files added, wait until there’s no more access to copy over whatever’s been added, as well as a copy of the database, second prompt once you’re done. Probably a bit of a pain for the users that host their files on a NAS while streaming from a laptop core and decide to go on a tour of Patagonia while mid-backup, but they’ll live and learn, and incrementals would probably help there.
This of course can be tricky for the subset of users that have more than what a single drive can hold, but if I’m not mistaken, we’re at 14Tb at this point, so that shouldn’t be too much of an issue, though a prompt about that would be nice (“hey ! we’ve noticed your library is 750 gigs. This drive is only 500, so please walk down to the store and get with the times”).
You could probably format that backup drive as well, maybe to NTFS for compatibility’s sake, on first sync, just to not have to have a unified base (a Mac user with ROCK couldn’t write to it, but that’s probably a good thing given the usage, I’m not certain there’s a filesystem with native, cross-platform RW that doesn’t come with massive compromises as far as a huge number of small files is concerned).
While we’re at it, if it were possible to hold checksums of the music library’s files on the external drive, that’d be nice as well, because it’d allow to know if and when there’s a case of bitrot, no matter how improbable that is. In a perfect world, and since the checksums wouldn’t be the same (one file has the added Roon metadata, the other doesn’t), store the same for the external drive on the ROCK / Nucleus.