My PowerMac G5 will not read my Leopard disc, dual layer dvd, how do I force boot from UBS DVD drive to install?? Or is there another way. It just keeps spitting out the disc!!

I have a power mac G5. Ir originally had Tiger on it. When I reinstalled for some odd reason the disc decided to install Panther, no clue why!! Either way I got a hold of a Leopard disc for this computer. It is apple certified and bought from Amazon and works for this computer.

Hoever, for some odd reason it keeps spitting it out. Discs under 4.7 GB is reads fine but the bigger discs it wont seem to read. What I need is to know how to get, or force, it to boot from an extrenal dvd drive. Or any other methods to get it to install.