I had tried another card, 8GB, and had the same problem.
But it seems to have cured itself after some rebooting of the computer. (Did I mention I had looked at the USB ports in Device Manager, showing no problems?)
The first thing I had checked was that it was in MSC mode, and I rechecked after reinstalling the firmware, so it wasn’t that (and there was no Fuze icon in Windows Explorer, which I think MTP would have produced).
I had been using a lot of different USB devices in that port, including another music player with USB slot. It, too, was working one moment with its USB slot, then not showing the USB slot, then showing it again. Maybe one of the USB drivers got jealous or something.
Anyway, it’s solved until it happens again. Some Windows glitch, I reckon.