Mac mini 2012 with Dell U2717D & Dell U2417H So I'm having an odd issue. I'm pretty sure this only started since macOS 10.12 but I can't say for sure.

I'm using a late 2012 Mac mini with two displays. The first display is a Dell U2717D connected via DisplayPort. Resolution is 2560x1440 @ 60hz. I've never had any problems at all with this monitor connected to the mini. The second display is a Dell U2417H, connected via HDMI at 1080p resolution. I only recently started using it with my Mac mini again, but I use to use them both together on the mini without issue.

If I start up the mini with both monitors connected everything works perfect. The problem only arises if I power down the HDMI second display or plug it in after the mini has already been running.

What happens is the DisplayPort main display loses it's signal. Nothing I do can fix it other then restarting the computer. I've tried unplugging / cycling power on both displays in every combination. Once the main display loses it's signal the mini has to be restarted. The HDMI second display continues to work fine. The mini still recognizes the main display but gathers the Display settings pane to the second display as if it knows the main display isn't working.

Anyone have any ideas?

Thanks.
There are two things I would try at the outset: Resetting your SMC and NVRAM. Instructions follow.

Resetting the SMC on your Mac Mini

There are a number of ways you can reset the SMC on the Mac mini. My preferred option on older Mac minis is to follow these steps:


Shut down your computer
Unplug it from power
Press and hold the power button for 5 seconds
Plug it back in and turn it on.


Note: If you prefer, you can also use Apple's latest steps for doing this instead. If so, ensure you're using the option for Reset the SMC on Mac desktop computers.

Resetting the NVRAM on your Mac Mini


Shut down your machine. Yes, a full shut down, not just logging out.
Press the power button and then press the commandoptionpr keys. You have to make sure you press these keys before the gray screen appears or it won’t work.
Hold those keys down until your Mac reboots again and you here the startup chime.
Let go of the keys and let your Mac reboot normally.


Note: When you log back in you may need to readjust some of your system preferences (e.g. speaker volume, screen resolution, startup disk selection, time zone information, etc).

Let us know how you go.