externals only work if there firewire not usb espically for an older mac, i upgraded to just a cd to a dvd on my emac cause im sick of putting it in target mode and using my g5 (brothers) to do the work and if u have 2 ppc u can buy a 6 pin to 6pin firewire cord and put the mac u want to install tiger on in target mode if that helps u any if u need more questions ask 😱