When you say 'USB Flash Drive' are you referring to one of those small USB 'memory pen' devices? If so, have you tried re-formatting it?
I use one also, they are great gadgets. I have one suggestion though - I always 'stop' the device before unplugging it. Even though it is USB and supposedly 'hot-swapable'. I'm on my second one, the first failing (full of data) after simply pulling it out.