Intel’s newer chipssets, like Intel HM75 are now including fastboot as a feature which allows the system to bypass some of the POST checks to speed up start up times. One of the issues with this is it also bypasses some of the boot up options like USB even when changing the boot order.
To solve this, boot into the BIOS and look for the Fastboot option. It may be buried in the advanced section. Disable the feature, save and reboot. Don’t forget to enable it again.