First, was it ever working or is this the first time you've used it? FAT errors can be caused by not having a microSD card in it, by not having the microSD card formatted, etc.
If you've used it before and then all of a sudden it stopped working with a FAT error, you could try reformatting the microSD card but you may have a bad AK2. If you have a bad AK2, you'll need to exchange it for a new one (most stores are aware of the problems with recent batches of AK2's and will let you exchange it).
-Bri