OK, thanks I guess I missed that! Then be sure you are not using that USB in a USB Hub, and that you are not using the blue USB 3.0 ports, use the middle ones under on top of HDMI and above eSATA, try both sets.
If you are unsure if your USB installer is setup right, test it on another machine, then you'll know for sure if it works properly or not. If it does not, do it again using Windows 7 USB DVD Download tool. I know you install from them regularly, but it's worth checking to be sure as it only takes 10 minutes or so to install from USB.
You do have Legacy USB Storage enabled in the BIOS correct? If not please enable this, it's on the Integrated Peripherals page.