30 minutes of angst when PC hung at boot due to external sata drive?

I recall couple years earlier gave my daughter an external Sata case with a 3.5 inch sata drive. I had tested the drive and had installed windows 7 on it in a different PC. I told her she can delete anything and start over with it.
So she brings it over, and it is working as expected. So this AM, I swap a PCI network card, reboot PC, and it hangs at the boot splash screen??!!

So start pulling cards, reset the bios, frankly was thinking the mb had just gone bad. So I start pulling connectors, keyboard, no, but the external drive was on and plugged into the USB and that was hanging the PC. I need to tell her to turn off the external drive.

So was their some boot conflict or why was it doing this?

Maybe it was trying to boot from it? And couldn't due to drivers etc being different.