It's definitely vfat. Some N810 had wrongly formatted internal memory cards (formatted for slightly larger than the physical size of the medium). As soon as Linux detects that (when it tries to write past the end of the medium), it should put the filesystem into Read-Only mode to avoid corruption. However, maybe in your case it didn't detect it but is still corrupting it? Try reformatting the card, which should solve the wrong-formatting issue.