So, I just ran into this issue. I’m at a client. Laptop comes loaded with Win 8.1. They have a Win 7 key they want to use on it. Normally, no problem. I carry a Win 7 x86/x64 USB stick around just for such occassions. Well, the Samsung didn’t want to boot from USB. I went into BIOS, turned off secure/UEFI boot, no love. Then, I find it. Under the advanced tab, there is an option for ‘Fast Boot’ … guess what, if that’s enabled, it DOES NOT SEE legacy USB devices. Disabled, reboot, BAM! Booting installer no problem!
EDIT – If you do this, be sure to use the RIGHT side USB ports, the left are USB3 and Windows won’t find your install media when you’re doing the setup.