if it is the internal card that's giving problems: does the tablet say "memory card available" when you replace the battery cover? If not, has the little magnet fallen out? it's a little square, 5x5mm, on the cover itself, by the latch. on the n800 it is, at least

if this magnet isn't there, the tablet won't notice that the rear cover has been replaced, and the internal memory card will remain disabled.