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I have a Mac Mini 2018. I have a main monitor which is connected using the HDMI output, and a second monitor which uses a Thunderbolt port with a Thunderbolt-to-HDMI adapter.

After working just fine for a couple of years, the second monitor started going dark, randomly, two or three times a day. Unplugging and re-plugging it solves the problem.

This does not appear to be a hardware problem. It happens with different sets of monitors and cables. I have sets of monitors and cables both at home and at work, and it happens in both places.

When the screen goes dark the mouse is still visible and I can move the white-outlined pointer around the dark screen.

This may have started happening after I did a recent Catalina upgrade, but I'm not sure. I'm currently running Catalina 10.15.7.

What could this be?

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    It's may be too soon to rule out hardware or software. The fact that it continues to do it with different sets of monitors only rules out the monitors being the issue. but the fact that the mouse is still visible points to the fact that the a video signal is being sent. Try this - take a screen shot and make sure that you select the blacked out monitor. Do you get an image of what was/should be there or do you get a black image?
    – Allan
    Oct 14 '20 at 15:31
  • @Allan Great suggestion. I made a screen shot of the dark screen, and yes, it worked. The screen shot contained the image that the monitor should have shown. What next?
    – ccleve
    Oct 14 '20 at 17:01
  • That means it's pretty much not an OS issue as it's generating the image. This points to a hardware (possibly a video driver/kext) issue. This could be the Thunderbolt to HDMI adapter. You want an active adapter when converting signals. Windows drivers seem to be more tolerant here about the signaling where as macOS isn't. If possible, you want to just use straight through DisplayPort connections rather than converting to HDMI.
    – Allan
    Oct 14 '20 at 18:17
  • @Allan Yes, it sounds like a video driver issue, because I'm using different Thunderbolt-to-HDMI adapters in different places, but they're the same brand. It's a QGeeM adapter. Will buy a different type of adapter. Unfortunately, my monitors do not have a Thunderbolt port.
    – ccleve
    Oct 14 '20 at 20:18
  • Make sure it says "active adapter" I've seen that brand and they are pretty much a "knock off" type company. Go with more reputable companies like TrendNet, CablesToGo, j5, Anker, etc.
    – Allan
    Oct 14 '20 at 20:49

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