I just ran into this problem yesterday. I believe It is related to the internal card becoming read only for some reason. To test, try to delete or add a file to the internal card.

The solution seems to be to reformat the internal card, but you can not do so if virtual memory is turned on. To turn off virtual memory, as root in terminal remount the internal card in with read write permissions:

# mount -o remount,rw /media/mmc2

Then use control panel to turn off virtual memory. You should now be able to reformat mmc2 via terminal, file manager, or as a mounted volume on a PC desktop.

You might want to save a copy of all those Navicore maps and any other unique data before reformatting.

One person who did not have root access via terminal simply turned off N810, plugged it into PC with USB and turned it on and was able to reformat the mounted volume that way.