More often, the failure to recognize a disk, or failure to boot to a DVD, is likely the fault of an older DVD drive, as those PPC systems are supported by the commercial Tiger install. The oldest supported PPCs - the G3s seem to often have problems booting to the Tiger DVD (I've tried most every Mac in my service shop). There is NO Tiger commercial installer that will install on any Intel Mac. The only disks that will work in that case, is the original disks that shipped with that Intel Mac. That's because the Tiger installer never shipped as a universal install, as Leopard now does.
And, that's not the posters problem.... My guess still is - it's not a FireWire drive, but is USB - and that won't work to boot Tiger on a PPC.