D520 keeps changing to external screen

I had posted this in the General  category a few minutes ago, then saw that there is a Video category.  

I have an old Latitude D520 which I have pulled out of my computer closet to give to someone in need.  It booted right up :) and then reminded me why I stopped using it.  The laptop monitor actually works fine but something in the boot-up switches it to "external screen only" mode (not monitor and screen), and thereby blanks out the laptop monitor.  I have tried switching it back using Fn-F8.  The switch works, but then it switches itself back to "external screen only" mode.  Any ideas?

Thank you for any assistance.  I have an extra screen that I could give them too, but it would be nice to have the thing still be portable.