Boot from an external hd?

I have a MacBook Pro and would like to install Mountain Lion on an external HD so I can boot from the external with my MacPro machine.
When I use the laptop to install the os onto the external drive will I gat the onscreen option of which drive to install the os?
I didn't want things to start installing on my laptop drive, just the external drive.
thanks.

Yes you can

As long as the drive is formatted as Mac OS Extended, it will come up as an option to install OS X on

If you want to control which drive you boot from each time the Mac starts hold down the Option key immediately after you hear the startup chime.

If you want to always boot from the external drive (sounds like it) go to System Preferences, open the Startup Disk pane and select the external drive. The next time your Mac starts/restarts it will boot from the external drive and continue to do so till you either change the setting or use the Option key.