So with leopard and SL retail disks I used to install a system on an external HDD (GUID, HFS+) and it would boot any Mac released before that system iteration (as long as all osx updates were installed). As long as you used the retail disks and not the grey disks from the box it would install all drivers for the existing Mac range. I had a 10.5.8 system on a partition and it would boot any leopard compatible mac. Same with SL and 10.6.8. Just alt boot in and you are there! I can install lion/ML on an external drive in the same way, however, the installer seems to check which mac is being used and only installs the relevant drivers. This means if you have created the system on a MBP it won't boot an Imac, just the exact same product family mac. Anyone know a trick to get round this? I like to carry my own systems around for troubleshooting hardware and software issues at work. Also if I have to use someone else's Mac I just plug in my 2.5 usb powered HD and boot into my own system, really handy for certain things.