Just a quick chime in, this thread scared me. BUT I bought a USB hub, and a sandisk usb drive (64gb), downloaded all my apple cloud music 99% of it in their m4a format, plugged all that into the Model S today and...it played just fine! No long indexing or nothing. Tried several songs, all worked with appropriate track listing just as if you popped the CD in, with appropriate artwork also. A real USB success!
I can duplicate this bug every time I get in the car in the morning. After the car goes to sleep, my USB music never resumes when I get in the car. If I open and close the Driver's Door a few times it may work but it's a silly thing to have to do.
This bug is linked to how long it takes the USB subsystem to initialize so should be quite easy for them to fix. I can confirm this bug is caused by a delay in getting the USB subsystem initialized when you first enter the car after it has been asleep. At home, I have Sentry turned off since the car is in a locked garage. If on my way to the car, I turn on Sentry Mode (which activates the USB subsystem for video storage), USB music works 100% of the time when I enter the car.
I tried switching to Spotify for music but the Spotify music player seems to have even more issues playing music. When listening to a playlist in random order, rather than play all the songs, it plays the same few songs over and over again.
The good news with the USB music player is that when it starts playing music, it works beautifully, though I wish they'd add playlist support.