I got the Sony Mex-BT3900U since one week now, but I experienced problems with the USB use.
In the beginning the USB function worked, but now when I step into my car and put my radio on and put my USB stick in the radio it says "No device" after a couple of times my it will see the USB but it wont play anything. In the beginning it was working fine.
I already tried to reset the radio, but that doesnt help.
My usb stick is 1 GB.
I have not yet tried another USB i ordered a new USB 16Gb USB 3.0.
This is very annoying, the rest of the features are working perfectly.