We are having issues with two Elitebook 850 G1 laptops running Windows 10 and using external adapters via the HP ultraslim docking station. It proves to be nearly impossible to come to a stable setup, either the external display is not working, either the laptop display is not working, opening the laptop lid does not work. In short, it’s a mess. Note that this was working flawlessly whit the same machine on Windows 7. Looks like a driver issue to me. Naturally the latest drivers are installed. I recently installed an Elitebook 850 G2 laptop and that one works like it should.
This type of laptop has two graphical chips: AMD and Intel. I notice that when I disable the AMD driver the laptop display is not detected. When the Intel driver is disabled the external display is not detected. Maybe someone can explain what is so beneficial in having two graphics adapters. And maybe it is possible to disable one of the two completely and let the machine work with just one? These laptops are not used for advanced graphical work, but their users would be very happy if they could use them with their external displays.
Have you updated the Docking Station Firmware? There have been a few releases over the years to improve compatibility with displays. WIndows 10 may "expose" new features that other Windows releases haven't.
Dual adapters are provided for performance/battery concerns. The thinking is the integrated adapter is used for most tasks and uses the least power/battery. The additional adapter is used when you need more performance/displays/features.