Hi, I found this step-by-step guide, of how to prepare a USB 3.0 key to be used to install Windows 7.
Apparently it enables the higher speed of USB 3.0 during the process.
Language is german, but I think the images show pretty good how it is supposed to work.
[HOW-TO] Windows 7 Unattended von USB3 installieren - ComputerBase Forum