I have a macbook with a shot internal screen, I took it out and am now using it only with an external display, the problem I am having is that it will run fine just after I install snow leopard but as soon as I reboot the external display will only show the desktop for a split second every couple of minutes as if it's trying to run off the absent internal display. Is there a way to set the external display as the main display so I can reboot my computer?