The mi box website states that it's supposed to support ExFat drives.

I'm currently using a 64gb USB stick as internal storage.(works like a charm) When I remove it and plug in an ExFat formatted 3tb drive it only lets me format or eject it. If I try a fat32 drive it actually let's me mount it as external storage. Has anyone else had this issue? If I format the drive on the MiBox it can't be read by my Mac. It's all a bit odd.. thanks for your help