It is pointless to solve your problem. If you succeed in creating the USB Windows installer, you probably will not be able to boot from the installer on your Mac. You are suppose to use the optical (DVD) drive. 

There are alternative methods to install Windows 7 on your Mac. For example, see the question: [Boot camp install of Windows 7 issue, no bootable devices][1].


  [1]: https://apple.stackexchange.com/questions/179486/boot-camp-install-of-windows-7-issue-no-bootable-devices