I realized that I have a USB connection in my car radio. So I put some music on my 4gig usb, I use for college. This played fine. I then wiped it all, and then put different songs on it. The new songs go on, but the old songs stayed? And theyre all messed up and sound like a scratched cd, they seem to mix up the tracks and change numbers? What is going on? How do I wipe it properly!?