Currently Being ModeratedJan 26, 2013 6:09 AM (in response to Darwindog)
You can't start from the Snow Leopard external drive. That's because Snow Leopard isn't compatible with the hardware of your computer.
However, you can try to install Snow Leopard on a virtual machine using Parallels, VMware Fusion or VirtualBox to run those applications. It's not the best way, but it's the only one to use Snow Leopard on your MacBook
Currently Being ModeratedJan 26, 2013 6:11 AM (in response to Darwindog)
No. In general, Macs can only be booted by an OS version at or after the one that shipped on the Mac.
The OS must support the hardware that exists on the Mac, and older OS's have no software to interact with the newer hardware.
Currently Being ModeratedJan 26, 2013 6:12 AM (in response to Darwindog)
Depends how new is "new". Users have confirmed that both early and late 2011 model MBPs will boot Snow Leopard from an external boot volume. 2012 models, probably not.
Currently Being ModeratedJan 26, 2013 2:45 PM (in response to Darwindog)
As per Mende1's suggestion: install your Snow Leopard (with Rosetta) into Parallels and run your PowerPC applications (which ones are they?):
[click on image to enlarge]