Prioritizing Internal SATA HD

[Cdn XxRRODxX]

Whenever I have my external Hard drive (connected via USB) on when I boot the computer, the operating system doesn't boot. However, when I turn off the external the OS will boot (the OS is on the internal SATA HD). Is there any way that I can prioritize the internal SATA HD over the external HD. I have already gone into the BIOS and USB is prioritized after the internal SATA HD so I'm not entirely sure what's wrong. Any help would be appreciated, thanks.

I am using Windows Vista 64bit and the external is a Seagate Expansion Hard Drive (2TB).

 

[olryst]

On the motherboards i've used if you have more then one harddrive attached it lets you set which one to boot from, which is separate from setting boot device e.g cdrom, usb, sata etc.

 

[prunejuice]

De-select BOOT FROM USB-HD in your BIOS as the first choice.