[freaktm]

After a lot of debugging, I found that the USB drive had locked up completely in read-only mode.
Trying a lot of different low-level USB tools, which should be able to fix the issue (resetting firmware, stuff like that), with no luck, I gave up and copied the contents to a new USB key. So all is good again.
Next time I reboot, I'll be a little more cautious with just pulling the plug on the Pi