You did not mention voice but this is what i was going to tell you ..... as you know when you put music onto a flash drive your computer via windows media updates your music files for extra information and this is how you get a picture of an album cover on the music folder.
The album art cover can cause you problems .... in my car i car play music from a flash drive without a problem but in my wifes car the music wont play because it cant "read" the data of the cover.
Try this as an experiment it works in my wifes car .... first put all your music from the flash drive into your "my music" folder on your pc , next delete all the music from the flash drive. Now go to the my music folder where your albums are. Left click on an album of your choice to open it past the artwork , your will now see another folder with the same name containing the music , right click on this folder and do ... send to flash drive.
Just in case it dont work for you just try it on one album as a test