Sometimes it all depends on when each of the computers were made, and whether or not they were released around the same time. There are some work a-rounds. For example my Original G4 733Mhz was not suppose to take or allow 10.5 to be installed, yet I found a way to install it by making my G4 A slave Firewire Drive to a G4 iBook. Your mileage may vary by model, but provided the is NO DANGER or DATA LOSS, I would try a few installs via this method. Good Luck!