Windows 7 USB/DVD Download Tool error: We were unable to copy your files. Please check your USB device and the selected ISO file and try again

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.

1. Start command prompt as Administrator and type  diskpart
2. type  list disk
3. type select disk  and number of your USB disk ( like select disk 1 )
4. type  clean
5. type  create partition primary
6. type  select partition 1
7. type  active
8. type  format quick fs=fat32
9. type  assign
10. type exit  to exit the diskpart utility
11. type exit  to close command prompt