I've read all the answers to this ( Sounds like a load of Guess work to me). Here is how to fix this problem quickly and easily.
I have a EMTec 32Gb flash drive, which every time I plug it in window's doesn't recognise it. The fix is very easy and it works every time.
Go the the computers Device Manager from within the 'Computer Managment' screen.
At the bottom of the list of devices is 'Universal Seial Bus Controller'.
Exand this window. You will notice that there is an unrecognised device in there. This is the key that you just installed.
Directly above this you will see a 'USB2 Enhanced Host Controller'. Highlight this and right click on the mouse.
Select the 'Unistall' option on this device.
At the top of this window there is little monitor icon with a magnifying glass, click on this. The computer will then scan itself and reload any devices and hardware it finds. It will also load the flash drive so that you can access it.