Can't boot Windows 7 USB Install on iMac 8,5
I think the problem is the way the computer is locked, if your computer has a Super Drive, your Mac is automatically configured/locked to boot from that drive and ignore the USB.

Their is no amount of reconfiguration or software tricks that would change that, what you can do is take out the Super Drive, or try cleaning it with a CD cleaner - the one with the brush.