Can't connect to LAN when booting MBP from external drive?

In an effort to always work in my computer environment, I backup my Mac Pro main drive to an external firewire drive, plug in to my new Macbook Pro 2.9 and option start off this drive. Result is I'm working off the last backup of my main Mac Pro drive so all my apps, stored mail, etc, is alive and well. I merely track any changes and new files and copy them over or use DropBox to get them on to my Main Mac Pro, my main computer.

Problem is, when booting off my external drive running Lion 10.7.5, my MBP's network won't turn on or recognize my home LAN. Then if I restart off just the MBP drive, it does connet to my LAN automatically.

Could the problem be that since my MBP was set up for Mount Lion, that it won't fully work with a drive with a Lion OS on it? I thought perhaps I had to turn on Airport when on my external Lion drive, since I don't have that turned on on my Mac Pro which uses ethernet. Can't figure this one out. The MBP must have airport as I'm on it now but booted off the internal ML OSX drive.

Thx,

Steven