My 770 with the unofficial OS 2007 shows a similar problem:

I cannot modify the filesystem on the MMC with the filemanager anymore.
The card is OK (fscked already), and it happens to every MMC inserted into the
770. The funny thing is that all applications on the 770 can read and write the card. Only the filemanager keeps telling that it's not writable.

I guess it's time to reflash the device...