M32R: USB Drive format?

Store bought FAT formatted USB drives seem to work (they are recognized and can be recorded to), at least small(er) ones. I tried FAT formatting a USB drive on my Mac but it is not recognized by the M32R. Can't record to it. I believe I've tried deleting the hidden MacOS files to no avail.

Are there any particulars WRT FAT format?

Do I need to firmware update my M32R?

Please advise :-)
-thanks ,mike