By default your Laptop's LCD is your Primary Monitor while the external LCD is your Secondary Monitor.
You can easily verify this in the Diplay Properties | Settings tab.
Click on the button Identify and it will clearly indicate this.
Also after you have selected the second display you have to check the option Extend my Windows desktop onto this monitor
The difference in aspect ratio will mean that your external LDC will not display the entire width of the Laptop's display.
I'm using my laptop in a docking station and a separate flat panel as my desktop set-up. Does anyone know of a good resource to help me set up for dual monitors. I'm having issues concerning which is the primary monitor. I travel with my laptop everyday and then dock it when I come home. Also my laptop has a wide aspect screen and my flat panel isn't. Any help would be appreciated.