Alright, so I'm back up and running now. I don't know why exactly.
I shut everything down so I could pull each drive and mark it's location on my spreadsheet. I checked the HBA, it and its cables seemed fine.
I went back to my list of 'missing' drives. None of them were on the HBA. All 8 drives were in one external USB enclosure.
I'm guessing that when I powered down the enclosure to get at the drive serial numbers, it reset something in it, and let unRaid see them all again.
I successfully upgraded back to 6.12.6.