Z unless I'm mistaken Win10 forces UEFI. In Win7 it was optional. They consider the not being able to boot from a USB device as a security feature. It keeps someone from walking up to your computer, shoving in a USB drive, and then booting your machine. If they do that then they can get to your HDDs.

As the machine is booting you should be able to go into BIOS and choose to boot off of the USB device. It has to be done manually every time you boot.

I ran into that on my I7 machines. I called ASUS and talked to tech support to find out how to get around it. They said there was no way to defeat the UEFI feature in their BIOS.