A clue?

Hm, the card reader started working again just now.

I had tried to copy a file onto the card while it was in the camera, over the USB cable, and the Explorer hung when I tried to access one directory—and I couldn't power down the machine from the Task Manager, so I resorted to switching off the machine at the power supply, then re-starting.

On coming back up, the card reader drives were there… maybe it just needs a hard start every once in a while or something, since the other times it came back up after a blue screen which (IIRC) also necessitated a hard power-off.