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.
I used Apple Lossless after watching a Tesla audio system review by Matt Farrell on his Undecided YouTube channel. As he said, it worked beautifully for months. Then the software update started giving me Loading Error messages. There has to be another reason why this is happening, especially since there are others who are having the same problem.