Aurora will not boot with external backup drive.

I've got a new Aurora and I bought an external hard drive over the weekend for a backup drive. With the ext. drive plugged in the system freezes when booting up. I believe it's either trying to boot from or with this external drive because I can unplug the extra drive or turn it off and the machine boots normally. Can someone point me to a setting I can change to keep this from occuring?

Thanks in adavnce.