In the BIOS of my ASUS motherboard there are two sections on the BOOT page. The first is 'Boot Disk Priority' which includes CD-Rom, Floppy, and the HDD which is set as 1st Drive in the second section called 'Hard Disk Drives'. This section allows you to position your hard drivers as 1st, 2nd or even 3rd if you have enough drives fitted. The 1st drive is obviously the one that will be used to boot from. You may have similar settings.