When you plug in the USB cable the N800 will first unmount the cards before making them visible for the PC (i.e. you can either access the cards from the N800 or the PC, not both at the same time). But if an application holds a file open on a card the N800 can't unmount it, and thus can't make it available for the PC over USB. It's the same if you have a swap file there: Can't unmount. You would first have to turn off the extended VM (in control panel) if you want the internal card to be visible to the PC.