So we are testing the Surface Dock with dual HP E221 monitors. We have several SP3s but only two SP4s the SP4s work fine with the Dock, After jumping through hoops to set resolution, but the SP3s aren't. The SP3 recognizes the monitors and shows them in the Classic Display Settings Control Panel Applet but they remain black after extending the desktop to them. The Surface has been fully updated with updates and firmware but we have had no luck. we know the cables are good because they work with the Surface 4 Pro. Here is the setup. Surface 3 Pro I7 256GB with 8GB Ram Window 8.1 Ent. Surface Dock 2 mDP to DP gold plated cables from Startech 2 HP E221 22' 16x9 Monitors As I said I everything works with the SP4 but nothing is working with the SP3.