I have had a look around and there are many things that
might cause your problem, USB too small for the amount
for the amount of music on it, or too large for the device
that is trying to read it, or a faulty USB, a virus of sorts,
+++, one video that I watched on YouTube, was interesting,
the person who made the video, like you did format it for
FAT32, but he partitioned the USB first, did you, if you didn't
then that might be causing a problem.
You could check in Bleeping Computer, they were very good
in the past, but I haven't used them for a long time.
Here is a link to a YouTube video, he is telling you how to make
you flash drive smaller, if it is too big for your car to read it, it is
informative and he takes time to explain how to.