I found the solution to that problem in this post.
You have to start command prompt as an Administrator (On Windows 7 that means right clicking the cmd and selecting Run as Administrator) and use the diskpart utility.
- Start command prompt as Administrator and type diskpart
- type list disk
- type select disk and number of your USB disk ( like select disk 1 )
- type clean
- type create partition primary
- type select partition 1
- type active
- type format quick fs=fat32
- type assign
- type exit to exit the diskpart utility
- type exit to close command prompt