Using snow leopard on external hard drive w/Mountain Lion OS Mac

Question

I need to use Snow Leopard in order to use some expensive software that will not work in Lion or Mountain Lion. All of our computers have been upgraded to Mountain Lion, whcih doesn't support Rosetta. I was able to install Snow Leopard on an external hard drive and it boots up fine from another computer (that is not mine) that has Snow Leopard as its OS.
But when I boot up from my MacBook Pro (running Mountain Lion) and from my Macbook Air (also running Mountain Lion), I can't seem to do it. I hold the "option" key when starting and I get the option to start from the computer's HD or from the external firewire Snow Leopard drive. When I choose the Snow Leopard drive, it crashes and gives me that black screen with the kernel (?) message in several languages that I need to hold down the Power key to turn off the computer and reboot.
How can I get my MacBook Pro or my MacBook Air to boot up from the Snow Leopard external hard drive? (Once again, it DOES work with a Mac that has Snow Leopard as its OS).

Answer 1

Richard Signes2 wrote:
The question is: How do I start up my MacBook Air (or MacBook Pro) from an external FireWire Bootable hard drive with Snow Leopard on it?  When I start up either one with the "option" key pressed, I get the option to start up with the external Snow Leopard hard drive, but when I select it, it crashes, as described above.
You can't if the Mac is a Late 2011 and later, perhaps if it's a Early 2011 (as some came with 10.6 and some with 10.7) and it should be possible for all Intel Mac's earlier than Early 2011.
Also, like what has occurred in the past, Apple does eventually issues a firmware update in later OS X versions so that earlier OS X versions, despite the Mac previously being able to boot from them, won't any longer. However far as I know Apple hasn't done this quite yet, but will one day.
Baring those issues, if you installed 10.6 on a external drive using a machine specific black/gray disk from another Mac, it has hardware drivers for that model/year of Mac only.
If you used the 10.6 white retail disks from Apple, it only has hardware drivers for all previous Intel Mac's that came originally with 10.6.2 or earlier from the factory. Mac's with 10.6.3 or later on the drive from the factory have different hardware drivers, thus not on the 10.6.3 white retail disk.
To find out what OS X version came on your machines, use the free MacTracker and use your model indentifier in the Apple menu More > Info (aka System Profiler)
So for those Mac's that originally came with 10.6.3 or later, machine specific disks must be gotten from Apple, or if your really talented, it's possible to create a univeral 10.6-10.6.8 install disk by combing the 10.6.3 image with the 10.6.8 combo update which will boot on any Intel Mac if the firmware permits and the hardware drivers are compable.
There is a method to bypass the firmware and install on Mac's that are Early 2011/Late 2011 via Target Disk Mode, or via taking the drive out of the machine as some Late 2011's use the same hardware drivers as the Early 2011 machines (only a processor bump basically)
How to revert your Mac to Snow Leopard
For the MacAir, you can install 10.6 via a USB thumb drive, of course provided it has the hardware drivers.
http://www.maciverse.com/install-os-x-snow-leopard-from-usb-flash-drive.html
So in your case you likely should be rather looking at running Snow Leopard in a virtual machine program in Mountain Lion, which MichealLax over at MacRumors has perfected a technique.
Windows in BootCamp or Virtual Machine?
http://forums.macrumors.com/showthread.php?t=1365439