Install 10.8 on an external drive from 10.9 I have OS X v10.9.
I downloaded v10.8 installer from the AppStore in order to install it on an external drive.
However, when launching the installer it says that it can't be installed on this computer and give no option other than quit.

What's the easiest way to do install 10.8 on an external drive when I already have 10.9 installed?

I'm aware that there are some answers about downgrading from 10.9, but I wonder if there is any easier way than the hard process described in other places for downgrading.

Edit: THIS IS A MID-2011 MBP so it did not come with Macericks pre-installed.
I would bet you tried to install Mountain Lion on a Mac that shipped with Mavericks. Unfortunately (for driver reasons) it is not possible. 

If you want to run an older OS X on your mac I would suggest try a VM like VirtualBox or VMWare.