About two firmware updates ago, the situation with getting "loading error" while listening to music off USB got a lot worse, it wasn't once every few days, it became a complete and utter fight every time I got in the car.
Queue a descent into misery, new USB sticks, forgetting I had to format them FAT32, find the utility for the new larger USB sticks since Windows won't do it for anything larger than 16Gb.
It would at least load the stick but about one song in ten still wouldn't play. It was merely annoying now and so it stayed that way for a few months.
I had some time today and I rang Tesla Support, some 60 minutes later I got onto someone, short version she had nothing useful to offer, said one to have one file type and that MP3 files worked better.
Will you cut to the chase already ? Alright, alright. Okay, so I proceeded to go through all the files on the entire USB (all 1760 of them) and came up with approximately 20 albums that wouldn't play. I tried to play them on my PC, they'd play on Winamp, VLC, Windows Media player, iTunes, irfanview and even my Sonos.
I then dug out my MP3 diags programme I had laying about and ran it over the USB, a few whinges about "poor quality" files (< 192k) but that's about it. I then had a closer look, all the files had "Ape" tags, every single one that wouldn't play. Hmm, a hell of a coincidence.
Some googling later and I found this page with a python utility to remove the Ape tags. I've since run it over all the MP3s and took it back to my Model S.
Every single one now plays.
[TLDR: OP rants about her music not playing and how Tesla's hate APE tags in their MP3 files, removing them is good]