Booting from USB drive?

I've got a 2007 20-inch iMac logic board with a broken internal SATA connector. I've tried epoxy, solder, etc. but I can't keep the part to stay on properly. Does OS X have to see an internal hard drive attached to continue boot, or can I have an external USB drive attached and have the system automatically boot to that drive upon startup without having to go through some special procedure? It's for a client of mine, he's old, and I don't want him to have to press a special key combination to get into his OS.

 

And I can do that with a USB drive instead of an external? Are you sure OS X will still boot without an internal HDD attached? For some reason I have a memory of OS X refusing to boot if there's no internal attached, maybe in the G4 G5 days.

 

Ah, I meant internal. And I keep pointing out USB specifically because I thought I recall know OS X prefers firewire over USB for certain operations (like target mode).

Thank you.

 

On some Macs, the USB ports are very slow compared to the firewire ports. Is this true for the 2007 iMac? I'm guessing I should stick with a firewire 400 enclosure for the boot drive; I want it to be about as responsive as the internal drive.