I have a MEX-BT3900u and am very happy with it apart from an issue were not all music on usb memory stick is seen by the unit.
I have a verbatim 8GB low profile stick that I leave perminantly plugged in, that currently has 70 top level folders and 504 folders in total containign 1219 files coming to 7.1GB.
The head unit only sees the first 51 folders, i've created a test with 150 top leve folders on a blank stick and they are all read.
I'd read somewhere that the format of the music is important, however, in my case music that cant be seen, can be when folders above it are deleted.
This is quite frustrating as I cant find the pattern to fix this, does anyone know what causes this?