1) remove the flash-stick and restart your computer.
2)while restarting press the F8 key multiple times until you see a menu appear. Then choose the repair option.
3)When it is done booting choose the Command Console/Prompt option, then type DISKPART and then LIST DISK.
4)Insert your flash drive an then type LIST DISK again. The flash drive should now be listed in the available drives.
5)Then type SELECT DISK (X), (X) is the number of the USB drive that you saw in step 4.
6)Type LIST DISK again and make sure the USB drive is selected (very important as you will be erasing the contents).
7) Now type CLEAN
8) Now type WPEUTIL REBOOT, this will restart your computer
9) Now use Window's RMPrepUSB, Quick Test to test the USB stick.
10)Now reformat the USB drive using Prepare Drive in RMPrepUSB
*Step 6 is very important. Make sure the USB drive is selected or you could wipe your entire hard-drive. Make sure you've selected the USB drive.