Booting from external drive

[A Marshall]

i'm thinking about getting an external firewire hard drive and putting a basic instalation of os 9 on it, just incase i need to do an emergency boot from it, my drive in my cube only has os X installed.  What i wanted to know was is there a specific key command to boot directly from an external drive or would i have to boot from a CD first and then select Start Up Disc?

thanks
~a.