Hi mark6188! Did you ever get an answer for this problem? Here it is, three years later, and my WD My Book is not letting my Dell XPS435 boot. Same problem as you described.
I bought a Wester Digital My Book 750 usb. I works great if I plug it in after the computer is already up and running.
However, when I leave it plugged in and reboot, the computer locks at the bios screen. All I can see is the HP Invent Blue screen that asks to go into setup or recovery mode. But the computer is locked up and goes no further.
When I unplug the drive, still nothing. I have to turn off the computer, unplug the usb cable, then turn the computer back on. After windows finishes loading, I can plug the My Book back in and it works fine.
I just did a bios update to see if that would fix it, but no. I have an hp m376n media center 2004.
I've also unplugged all usb devices except the drive and the computer still hangs on the bios screen.
I guess I can live with plugging the drive in after the computer starts, but it's a hassle since windows update is always rebooting my computer. Then I find out my favorite show didn't get recorded. ARRRRGH!!!