I am a N8x0 newbie and my Unix/Linux knowledge is thirty years out of date, so take this for what it's worth.

But when I've seen the "Memory card in use" message, it's been because I had an app or dialog open that referenced a folder or file on the card.

Have you tried turning your unit off and on (i.e., rebooting)? I've seen one or two strange repeatable behaviors on my N810 that have been cured by rebooting.

If that doesn't shake it loose, see what processes are running. Maybe someone can post, for comparison, a list of the processes running in a clean OS 2008 with no apps installed.