If I have USB bootable media plugged in, SeaBIOS seems to immediately boot from the hdd. I'm not able to boot from USB unless I physically unplug the SATA connection.
I've noticed on my system at least the USB device doesn't show up in the boot menu on a cold boot. If I hit ESC, then ctrl-alt-del, then ESC again on the warm boot I can select it. Also, try a USB 2.0 port instead of 3.0 if you have one.