60sIron

I have a 16Gb USB flash drive with mp3s on it that used to play fine, then I loaded some more songs on it, now it plays for 10 seconds, and then says it is "unreadable". The drive seems to check out fine on a computer. I'm guessing there is a file name or a folder structure MMI doesn't like. This is my first Audi, so I don't know the rules. Does anyone have any tips on how to structure music on a drive to make it "readable"?