This issue has driven me to distraction with the last 2 Asus boards I had, where if anything was changed in the drive section (and occaisionally even when it has not) then the BIOS would select a brand new drive for me to boot from - one without the OS usually. Safest situation to stop it is to have your boot drive attached to the first physical port (i.e., last computer was IDE0, this one is SATA0). Anything else and it will rearrange drive priority for you at some point. Not fatal but annoying.