Have you checked to see if the magnet on the cover is missing? I had mine go missing and it caused the same problem. The N800 uses the presence of this magnet to indicate when the back cover is on and only initializes the internal card slot when it registers the cover as being closed. Try waving a powerful magnet over the backside of the n800 and see if you get "memory card detected" on the screen. I ended up epoxying a magnet from a pair of crappy earbuds to the inside of the back cover to fix mine.