The secret sauce, and the reason I'm even writing about them, is over on the "service" side. Their machine has a built-in phone-home feature that automatically duplicates the drive off-site -- at Datto's secure, redundant, automatic fail-over data centers. For $25 a month (they have some pre-payment discounts), the NAS contents are automatically backed up off site. The fee is per box, not by volume of data. It's obviously smart enough to do only bit-level back-ups so, absent the initial data load, the bandwidth required to accomplish backups isn't significant. And, if you have a large data store (a few hundred GB can take a LONG time to load!) you can ship your box directly to them initially and once they return it you're off to only bit level again.

I'm not certain that this is the be-all and end-all solution for small tax and accounting firms --- but it certainly DOES solve most, if not all, of the related problems.