I tried booting the external firewire hard drive after my internal hard drive died, with out success. I tried holding the "T" key, option key, space bar, and any thing else I could think of. A symbol, circle with 3 legs at 120 degree separation, would appear on the screen and float around. Did the same on my working Mac when "T" pressed. Had to go to system preferences and select the external drive as the boot drive to boot from it. This was not possible on my failed Mac .
Is there a way to boot from the external drive when the internal drive is kaput?
P.S. Failed Mac would not recognize optical drive either.