Not sure how I screwed that up so much. Previously I had successfully installed Debian to boot from an external HD. But now I’m repurposing it to do something else, so I was going to reinstall a fresh Debian image to the EMMC.
The bios Boot options menu only lists 1 item, the internal UEFI. Doesn’t list even the onboard EMMC.
The bios Advanced options menu… USB configuration menu does list the USB drive that’s attached, so it’s not as if the the SBC itself can’t detect at least that.
Resetting to optimized defaults doesn’t clear this condition.
What else can I try?