Sometimes my N800 gets fussy the same way, depending on what's been running recently (background or foreground). It usually clears up eventually, but who wants to wait? I found this workaround:

Try shutting down the tablet, then making the USB connection to your live PC and restarting. That's always worked for me, no matter what. Sure, it's a few extra seconds, but it beats randomness . . . in fact, the memory cards show up in Windows Explorer even before the N800 splash screen clears.