I made the same mistake last time. I thought my Emunand is dead, and so I just experimented around. After I just backup the data (aplain copy, no fancy partition image creation), i tested it with FAT32 and it worked, beside it wanted me to reinstall everything. So I did just another thing: I just reinstalled the firmware update, this time correctly as ExFat and oh wonder, everything was working again. I don't guarantee that it will work, but as long its just Emunand, the danger of bricking anything should be low.