i prefer to be absolutely 100% certain that nothing go's wrong (even though it did the first time >.<) i actually wanted to physically unplug the drive's before i noticed i could do it via the bios :P

only got this mobo recently and it's not a feature you'd use often unless experimenting with a new os hours after it's become available and seemingly wipes your pc clean if you screw up :P

if your motherboard's bios allows you to select which drives are active (the sniper M5's have this ability i believe most have).

save your current bios profile (mine's called windows)
then just dissable all hard drive's you wanna save via the bios and save the profile (i called mine linux since that's what it will be used for)
save and exit
run steam install on the sacrificial drive
remove usb stick at the end as the screen will tell you
load up the windows profile and there ya go.

dual boot, i generally wait untill steamOS is fully installed before i re-enable my windows drive's just to be safe.

did spend a couple hours trying to fix the MBR the first time around though, the usual fix wasn't working >.<

i sit here writing this from steamOS because of you, thank :D