The N800 doesn't make the card available for USB unless it can unmount it (i.e. make it invisible for the N800) first, and it can't unmount if the card is in use. So, to keep one card mounted on the N800 and not available for the PC you could keep a file open. You could write an application, but the easiest would probably be to keep an .mp3 file or something on the card you want to "reserve" and start playing it just before you plug in the USB cable!