I'd actually yank the external MMC, plug it into a card reader, and see what it looks like, then also scandisk it while it's out. If everythings fine and several unplug/replug cycles show it hasn't changed, or it changes correctly, and you're using an adapter with a microsd card, then switch adapters, reinsert the microsd card into the adapter, making sure it's solidly in place, then reinsert it into the tablet.

If that doesn't work, I don't know what to tell you.