I've just upgraded the hard drives of my two Macs. New (blank) HD into MacBook [then Time Machine restore of Leopard] >>> Old (OS 10.5) MacBook HD into Mac Mini (OS 10.4) Basically, the Mac Mini was running 10.4 Tiger. I inserted the MacBook's 10.5 Leopard HD into my Mac Mini, turned on the Mini to begin a fresh install of Tiger to my surprise the Mini was running Leopard. I had expected an error message. Firstly, I didn't think this was possible. How has it worked? Secondly, is was I have done legitimate? How long can I run the same version of Leopard on my Mac Mini and MacBook at the same time? Thirdly, should I just shut up, keep quiet and get on using them both? Sensible help appreciated.