Two days ago the worst thing happened: I tried to access the NAS over the network and couldn't.
I could log in to OMV ok, and interrogated the setup only to discover both Main and Backup filesystems weren't mounted (or mountable) and even the disks themselves were there one minute and gone the next. Everything was very flaky for a while before eventually I had nothing. Occasionally one disk could be mounted for a few seconds and then gone again.
Suspecting (wishing?) the SATA-to-USB adaptors to be the cause, I plugged one of the disks directly into my Windows PC to discover a perfectly healthy disk with an empty, unformatted partition! Eeeeeek! Thankfully, after some searching, it seems Windows doesn't like something about Linux partitions - something to do with GPT, EFI, or whatever...
Plugging the disk, via the duff adaptors, into an old Linux laptop I have confirmed it: one of adaptors did pretty much nothing, the other one repeatedly power-cycled the disk - as evidenced by dmesg logs. For a brief moment, it actually worked, the disk mounted ok and I saw my files! Phew! ...and then it was gone again.
So, two new (much more expensive, branded and guaranteed) SATA-to-USB adaptors later, and everything is back to normal. The lesson: don't buy £5 adaptors from eBay - they are cheap crap that don't work for long.
[PS. I cracked open the case to find a board with a chip marked "JMS578". Comparing the markings on the chip and on-line photos of the genuine JMicron part makes me think this chip is a fake - the typeface isn't the same and it's been laser-etched on. Maybe I'm just paranoid.]