Inexpensive NAS for over 4TB backups - recommendations?

Yes. One caveat is that the RN102 is using btrfs as the file system. I have had no issues with that, but it is a bit cutting-edge for operational use. It's snapshot feature is quite different from the Pro, and in my opinion more useful. I'd leave the anti-virus scans off (doesn't make much sense on the backup, and slows things down).

Also, OS-6 currently lacks disk spin-down. My guess is that they will add it shortly, but of course that gambling on futures.

dhl wrote:
...So if I understand you correctly, best to set up as a multi-volume JBOD, and schedule and run Frontview backup from the RN102 rather than the other way around (scheduling and running from Pro)?

Yes. I always run the backups on the destination NAS, it seems better especially if you are combining with scheduled power-on. As always, be careful to not to be aggressive on power-down, since the NAS will shut down even if the backup is not complete.

BTW, the RN102 also has WoL, so you could alternatively boot it up from one of the Pros on schedule. I haven't played much with that feature.

dhl wrote:
Is there any safe way to create a single large volume on a two-bay system like this, so I can avoid having to manually split data across two volumes?

RAID-0 is the only way. If you are wanting long-term archival, it is not safe. If you are just wanting a current snapshot of the pro, it might be good enough.

I recommend a UPS in any case, but especially if you go with RAID-0.