Having the exact same problem w/ a 4GB Sandisk. It worked for a while, now nothing I do makes it work, though it works in every other device under the sun.
A lot of you guys are having similar problems, so what firmware version is being used? Version seems an important issue with faults experienced on this deck as they are ALL BETA leaving it up to us to find at sometimes critical times.
Also VERY important to ALWAYS reformat the cards whenever taken out of the deck, and VERY WISE to reformat after deleting files as not doing this makes some decks 'trip' over leftover file fragments and discontinuous memory space.
Sometimes completely SHUTTING DOWN THE DECK, REMOVING external power, removing the MEMORY CARD, and then REINSERTING CARD FOR REFORMAT PROCESS IN THE DECK can work magic when nothing else seems to work.