I have a new macbook pro 2,6 Ghz i7, 8GB 1600Mhz internal ram. He will not boot from an external disk with operating system Snow Leopard

Maybe it's not compatible with Snow Leopard?

I could be wrong.

Restart your Mac while pressing the Option key. You will then be prompted to choose your boot drive. Choose the external disk that contains SL.

Hope this helps.

These Macs shipped with 10.7, and will run nothing older than that:

No version of Snow Leopard has the correct drivers for the hardware in the newest Macbook Pros, or any newer Mac (2012 models). So you will never get Snow Leopard to boot on that system.

It worked with both early and late 2011 models because at one time the early 2011 models came with SL Pre-Installed and the difference in hardware between the early and late 2011 models was not that different. Now with the Mid 2012 models it has a completely different chipset and graphic chip/s in it, both internal and add-on graphics chips, so SL does not included, and NEVER will included, the needed drivers for the newer models.

Spiderman3 wrote:

I hope you're wrong. It would be strange that this is not mentioned by Apple in the specs.

Apple has never allowed the installation and or booting to older OS X versions except in rare occasions like with the early and late 2011 models.

So there is NO reason for Apple to meantion it in any Specs. The Mid 2012 models will run either Lion or Mountain Lion when it comes out. But it will never run any older version then Lion of OS X. Not going to happen.

Spiderman3 wrote:

When I try this, my mac freezes. :-(

Right READ my posts above. SL will NEVER boot on that Mac.

I hope that is now clear to you.

Good Luck.

Yeah Apple uses the word Genuis a little to loosely, if you get what I mean.

You are welcome. Good Luck.