I got a 16gb stick in mine fat32 mp3 occasionally it stops working looses the usb, plug usb back into the computer run check disk fixes it.
this might sound odd but are the mp3 files rights protected to your computer only?
other thing maybe the bit rate is not comparable 128kb/s works fine for me.
if your worried about a size detection issue partition the drive and set active partition to smaller size yes you can split a usb
don't forget about the max number of files in the primary directory as well can't remember how many off hand but if there are too many there the drive won't work put the mp3's in folders i30 will read and play from the folder