I did take it up to 2.1.12, and tried to go back to 1.7.5. My fault, 100%, my 4 year old wandered down (well, ran) and clobbered the cable.
Booted up, got 'Er' and some number, and then 'boot looped'.
So right now I've got a brick. Going to try and see if I can force it to start by holding down various buttons and running the update software. It seems to know there is a device connected even if the machine shuts off, so (since it's an ARM device) it's probably the boot loader running in the background. That's good, means it might be recoverable.
Thank you for pointing me to the software and I bet it would have worked if I didn't try to yo-yo.