I'm having the same problem. I had about 40GB of music on USB from iTunes library with no problems until a software update. Now it won't play anything from USB. The files all show to be there, but I get the "Loading Error" message instead of music. I tried erasing the drive, then reloading a few songs to see if that would work. Nope. I still get the Loading Error message and no music. Help!
I forgot to mention that I was using a USB hub so I could use three drives; music, sentry, phone charging. I noticed the hub and drive were getting quite hot, so I removed the hub and tried plugging the USB drive directly into the car before reformatting and reloading the drive, but that did no good.