Secondary monitor connected to Satellite L775 becomes primary display unless monitor is disconnected at startup

[wltaggart]

I have secondary monitor connected to my Satellite L775 laptop, running Windows 10, and it works properly most of the time except when I start up the laptop with the secondary monitor video cable still plugged into the laptop video adapter. In such case the next time I start up the laptop, the screens are reversed, meaning the laptop display becomes the secondary and the secondary monitor becomes the primary display.
The only way I can correct this problem is to shut down the system, disconnect the secondary monitor from the video adapter on the laptop, then restart the laptop while the secondary monitor is still disconnected.
After the laptop has fully booted up, and the home page is now on the laptop, rather than the secondary monitor, I then plug the video cable from the secondary monitor into the laptop video adapter outlet and screens are displayed as I wish them to be.
It is not impossible to work around, but it is annoying if I have forgotten to unplug the secondary monitor video cable.
Your help will be very much appreciated.

 

[hang-the-9]

Why would you do all that, just go into the display options and set whatever screen you want as the main one. You don't need to reboot.