On my 810 this has happened twice now. Not sure if it's something I've caused or something that was installed. The aggravation of it almost makes me want to give the IT a heave out into the lake (just kidding).
Anyway, I've used the File Manager/Tools/Format Memory Card, cleans things up right away. But what the heck is causing it?

Other than that I'm very impressed with the N810 and its possibilities.