Control boot disk with Phoenix Award Bios 6.00

Could someone please help me solve a problem that's been making me tear my hair out for some months! I'm using a USB hard drive for backup purposes and would like to keep it permanently plugged into the USB port and powered up but each time I boot the PC it tries to boot from the USB hard drive and logically enough won't boot until I unplug it and power up again. How do I force the PC to look first at the main hard disk and boot properly?

I'm using Phoenix Award Bios 6.00P and have tried all the permutations I can think of around the Primary Master etc. none of which seem to make any difference.

In the advanced tab I have First, Second and Third boot device disabled and Boot Other Device enabled. If I disable Boot Other Device then I can't boot at all. Or not that I've been able to work out so far. All very confusing for a novice!

Regards
Simonides