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I have Mac Mini 2010 which has a HDMI and Mini Display port with adapter to HDMI. It has macOS Sierra 10.12.6 clean installed on an internal SSD. Nothing else is attached except an external USB drive.

Dynex DX24E LCD is used as the monitor and Dynex BBY LCD 26” MDP LCD is used as a home theatre display mirrored, but only part time. External speakers are attached to the headphone port. I don’t want to use HDMI audio at all.

Originally this ran flawless for 7 years on Mac OS X 10.6.8, only a couple years with second display.

When switching the 26" display on, the 24" display would freeze and go dark for 2-3 minutes. When it comes back on, the audio was being sent to the 24" HDMI. Checking the sound preferences panel, it was set to headphones.

Resetting the SMC and NVRAM initially makes the headphone audio work again. But switching on the second display - no audio even though its set to headphones. After unplugging the 26" display, which sound immediately comes out the headphone and waiting and replugging. I eventually got it to work, sound stayed on to.

Then after turning off the 26" display and a delay of 1-2 minutes, the 24" display would work fine. I leave the Mac Mini powered on but turn off the 24" display at night.

Same thing went on for 3-4 days, then It was back to normal with switching time of 10-20 seconds. Turning the 26" display on same thing pretty quick. Its been running fine for a couple weeks until I had to restart to gain access to something on the old system. After rebooting I get no audio and unplugging didn’t work.

I performed a fresh install of macOS Sierra 10.12.6 with the same result.

I did not try to Reset the SMC and NVRAM but swapped the display ports. Sound works but back to slow switching of 2–3 minutes.

I have the following issues:

  1. How to make it work without doing everything every time.

  2. What could cause ethernet port to connect/disconnect?

If I boot into Mac OS X 10.6.8 it works fine. When I shut down and startup macOS Sierra again, the ethernet port goes from connected to disconnected about every second or two. If I restart again the ethernet port is fine.

I have tried about everything a couple times. Read dozens of docs.

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