ZFS Pool Unavailable after upgrade 9.3 --> 11

[FNASFan]

Setting this jumper on a populated drive probably won't damage your data but it will definitely make it unavailable.

I'll save the Museum of '00s computing a call then.

Right now, I would advise you to stay on 9.3, where your pool works, and try to figure out a way to back up your data. Once that's done, install 11.1-U6 fresh, and create a brand new pool from scratch (which should be ashift=12) and then restore it.

I think you're bang on the money. I've got everything backed up on and offsite, so this is definitely an option - I'd got a bee in my bonnet to figure out what was up. As a post-script - I'm pretty sure this pool was originally created in April 2011, so it probably wouldn't hurt to create a new one! :)
@HoneyBadger and @Johnnie Black : many many thanks for your time. This is the first time I've gotten so stuck I've hit up the forum and you've shown it to be a well informed, helpful and friendly place. Credit to you both.