I had this same problem today, as a matter of fact. I couldn't figure it out at first. I plugged in the usb cord and it immediately began to synch... except it ended up erasing all the music on my iPod. I freaked out. All my music was still in my iTunes library but I didn't understand why I was getting the error message that the songs were in the wrong format (.mp3) and couldn't be copied to my iPod. The first time I tried it today I was using a usb cord that I usually use to listen to my iPod in my car. I took a chance and switched to the usb cord that I usually use to charge my iPhone and ... voila! That worked! I was able to sync the songs from my iTunes account to my iPod. Both cords are authentic Apple products but the 2nd one is newer so maybe that was it. In any event, that's what worked for me.