Many motherboards do not have a UEFI motherboard, this is a problem when installing Steam OS because it requires a UEFI. I have found a way to install Steam OS without UEFI.
Steam Database: http://steamdb.info/blog/35/
Reddit Post: http://www.reddit.com/r/SteamOS/comments/1su4t1/uefi_requirement_with_steamos/
I AM NOT RESPONSIBLE IF YOU MESS UP IN THE INSTALLATION AND END UP WITH AN EXPENSIVE METAL BRICK.
But feel free to message or comment and we can try to help :)
This is a re-upload of the video i uploaded yesterday that wasn't in 1080p HD.