how do I get automator to specify a startup disk?

I want to automate the specification of startup disks as I tend to frequently swap between a few systems. I fired up automator, hit the record button and proceeded to select the system preferences pane from the apple menu, selected startup disk pane and then clicked one of the disks. Automator listed my steps and it looked fine. However, when I play the script, instead of clicking once on the system preferences menu item it clicks twice on the app store menu item, resulting in an error. Obviously, I'm doing something wrong, so I'd appreciate some help. Thanks.