Use the "Expert Install" option
We have some good news if you've been wanting to experiment with Valve's SteamOS but have been reluctant to install it on a dedicated machine. Valve engineer John Vert has made available to download a new SteamOS beta build that supports dual-booting . The updated SteamOS ISO can be used to install Valve's Linux-based OS on non-UEFI systems, though keep in mind there could be issues with the build.
"PLEASE note there has been very little testing on this, especially any kind of dual-boot setup," Vert warns. "So don't install it on any machine you are not prepared to lose."
You can download the ISO here . To take advantage of the dual-booting feature, you need to select the "Expert Install" option, which allows for custom partitioning.
Results have been mixed so far. One user on Steam's forums commented that he was able to install the updated build in VirtualBox and that it " works perfectly, " while another user said his system simply boots to a black screen when selecting the SteamOS option from his dual-boot environment.