Changing the primary monitor to be DVI on Mac

"With an Apple laptop you can run the computer using only an external monitor by plugging in the DVI cable, and an external keyboard and mouse. You can plug in the the keyboard and mouse and DVI cable and use the keyboard/mouse to wake up the computer with the lid closed - the computer will then only use the external monitor.

You can find more info on the Apple KBase Article: Apple Portables: How to use your computer in closed clamshell (display closed) mode with an external display"