if its sata then you should be able to start up your main PC as normal and then plug the drive in after. windows should detect it as a new drive and install it (i'm thinking similar to eSATA). that way you avoid it booting fromt he laptop drive by accident
He try to do that but he need a other lead for the 2nd HD so that he can use them both at the same time and set the one out of thae laptop as the 2nd HD