Cant boot with external drive pluged in

I recently purchased a new desktop computer running Windows 7. I have a 500G Seagate external hard drive that I use to do scheduled backups of my data. The computer will not boot with the external drive plugged in via USB. When I unplug the usb, the computer boots with no problem. In the bios boot sequence, the order is (1) internal hard drive, (2) optical drive, (3) network driver, and (4) USB. The bios does not allow me to disable the USB on startup. I spoke with tech support from the computer manufacture. They told me all I have to do is plug in the external drive whenever I want to back something up. Ya right! Is this something that people are dealing with or should I send this box back and try someone else's. I would like to keep this one since it is such a pain in the a** to re install everything.