It's been about a month, so here's an update of my situation:
I still can't access the NUC's BIOS. I'm still running BIOS version 24 - I've downloaded version 25 but haven't tried it yet, for fears that it may lock me out completely. The release notes don't indicate that it allows the resetting of the "detect USB" settings and I've read of other people having USB problems after installing it. So for now, I'm not touching my running system :-)
That said, the machine is happily running freeBSD 10.0 and seems to be settling down.
Since my last post I've changed the configuration of the USB drives to be always on (i.e. not in low-power mode) and I wrote a script to randomly write and delete files to fill up the disks completely. It seems that ZFS has found all of the bad blocks now and seems to be running stably. The USB drives also haven't "disappeared" for about 3 weeks now. So it's looking good (although it would be better if the low-power settings would work, I might try them again soon, now that everything seems to be settling down)
I am still hoping that there will be an update to the BIOS which lets me reset the "detect USB" settings.
Sylvia? Any news?
Thanks in advance,
Steve_R