Hi there, I had the exact same issue as you, tried everything!
But still nothing worked.
In the end I found out it that it was either one of these:
A virus has entered the USB stick and corrupted it (Didn't seem very likely)
Or that the USB drive itself is faulty.
Turns out mine was faulty.
If I were you I would get all data off of it and then if in warranty return it,
I'm sorry but there not really any other way around it.