Mr. Keeks is correct, but note that you have to have a valid system folder on the external drive. If you haven't done so already, the easiest thing to do is just clone your internal drive to your external, and then just start up from the external.
My personal fave program to do this is SuperDuper!
Also, keep in mind that you'll have the best results with a FireWire hard drive. A USB hard drive will work as well, but it may not show up or be as fast.
Also, keep in mind that you'll have the best results with a FireWire hard drive. A USB hard drive will work as well, but it may not show up or be as fast.
My experience with Firewire with PPC Macs has been extremely positive/virtually flawless (both Classic and OS X).
My experiences with USB with PPC Macs (both Classic and OS X) made me think that it was really really flakey/a pain.
This has changed with the Intel Macs, where the USB works just as well. Not sure why, but I would always recommend Firewire when dealing with external disks with PPC Macs.