Hi ribo. I think it depends on the BIOS. The ASUS A7V8X-X, for example, gives a choice of first boot device that includes selecting which hard drive is the bootable one. Other motherboards only give a choice of hard drive, floppy drive, cd-rom etc. personally, I only ever put the bootable hard drive as primary master, so I don't know how the BIOS handles the drive letter allocation.