Over the last few weeks I've noticed a issue with my external MMC on my N810.

I can connect my N810 via USB to my computer. I can see the card with no problem. I can move files, erase, etc. Everything looks normal. When I remove the USB cable, and view the card on the Nokia in the file manager, it looks as it was before (without the changes I did on the computer).

I open File manager, and erase files from my external MMC. It looks fine (like they are deleted). Close file manager, and reopen, and it shows the files are gone. Connect via USB, and check on my computer and the files are there.

Should I reformat my external MMC?

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.