This is quite an old discussion, but in case it helps someone...
With my GA-Z77-D3H motherboard, I tried a bootable USB in the front USB port(s) and couldn't boot from USB (tried memtest86+ and then debian installer) and then later on, I found that I could boot from the rear USB connectors. It seems that there is an internal USB hub and not all USB ports are created equal.
More recently I was able to boot from the front ports, but I had updated the BIOS a couple of times since the first time.
So if you get the same problem, update the BIOS and try booting from the rear USB ports too.
If you have a recent motherboard you need a Linux image with the latest (or at least recent) kernel/drivers.
In my case, I installed Debian 6.0 (aka "squeeze") using the backported installer image squeeze-custom-amd64-0315.iso from:
http://kmuto.jp/debian/d-i/