I had the same issue with the same card... I tried running the Unix "du" command on my memory card and received errors with many files in the "System Volume Information" directory. (This is a hidden directory created by Windows.) I tried to delete the files in the directory, but received errors about the card being a read only device or something like that. I decided to reformat the card and tried to do it from the 770's File Manager, but received an error. At that point my memory card was not accessible. I then connected my 770 to my desktop (Windows XP), was prompted that the card was not formatted and asked if I wanted to format. I formatted the card and everything has worked great ever since.