It failed again and this time I was able to connect a monitor and keyboard to it while it was “down”. Wasn’t able to type anything, but I could change TTY. They either showed blank, or one of these 2 (with new messages being sent every 10 seconds or so):
So I’m guessing this confirms a USB boot drive issue. I’ve tried my NVMe-in-external-USB-enclosure in 2 of my 4 USB slots (one 3.2, one 2.0) to no avail. I’m guessing trying the other 2 isn’t going to help.
I have a USB flash stick arriving tomorrow, so fingers crossed that works better than my current solution.