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I have a Mac Book Pro (latest 2017 touchbar model) which I am using in clamshell mode. Meaning: its lid is closed, and I work with two external monitors.

Both monitors are a bit older; they have DVI ports which connect via DVI/HDMI cable. Monitor 1 is then connected via a Kensington USB-C dock, Monitor 2 is connected via a StarTech HDMI<->USB-C adapter.

My problem: as soon as I lock my screen ... Monitor 2 will stay black. Meaning: I come back, I log into the system ... Monitor 1 works just as before. But Monitor 2 stays black. Although MacOs somehow still thinks the monitor is "there": when I turn to "displays" and look at "arrangement" - it shows all monitors. And windows that I "left" on the desktop space on monitor 2 won't show up on the other monitor.

I guess this might be caused by the chain of hardware I am using - but still it is extremely annoying to come back and find that monitor to be useless (I have to restart MacOs in order to get it back working).

Any ideas how to overcome / debug this issue are welcome.

( edit: same thing happens when I "sleep" instead of lock )

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An easy thing to try is resetting the NVRAM on your MBP.

To reset the NVRAM on all Late-2016 and 2017 MacBook Pro models (with and without Touch Bar) follow these steps:

  1. Shut down your machine. Yes, a full shut down, not just logging out.
  2. Press the power button and then immediately press the commandoptionpr keys.
  3. Keep these keys pressed down for at least 20 seconds!
  4. 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).

If you find the above makes no difference you could try booting into Safe Mode and testing to see if the behaviour is repeated while in that mode.

Boot into Safe Mode

Follow these steps to boot your MBP into Safe Mode:

  1. Fully shut down your MBP
  2. Restart your MBP
  3. Immediately press the Shift key and keep it down
  4. Let go of the Shift key when you see the login window (NOTE: If you have FileVault enabled you may need to log in twice).
  5. Take a note of what happens (i.e. use your computer as normal and then lock the screen, etc)
  6. When you're finished exit Safe Mode by restarting your MBP as normal
  7. Test again while booted normally

Let me know how you go - in particular whether the issue still persists in Safe Mode.

  • At least the "reset NVRAM" did not help. After doing that, Mac comes up, two monitors ... clicking on "lock" ... bang, only one comes back. Probably the underlying reason is the fact that monitor 2 is years old, and has only a DVI output. – GhostCat Oct 10 '17 at 12:27
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I know this question is almost a half year old. However I think it could be useful to someone to know, how I solved this problem (black screen on external monitor after locking). In my opinion this problem is not exclusively related to Apple or MacOS. I had the same problems also with Windows 10 on a Dell XPS 13 9360 laptop. My docking station is a Plugable Thunderbolt 3 Docking Station with two external monitors connected: One via Thunderbolt 3 to HDMI cable and the other via Displayport to DVI cable.

My solution to fix the black screen problem (effected my second monitor connected via DVI) was to install the newest DisplayLink driver:

  1. Disconnect the docking station
  2. Execute the DisplayLink Installation Cleaner.
  3. Install the latest driver from the official download site. Drivers for macOS, Windows, Android, Chrome OS and Ubuntu are available.
  4. Connect your docking station again
  5. Reboot the system
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When i faced this issue , I pulled in and out the StarTech Adaptors multiple times and the issue used to get resolved. Now i found a better solution as described below.

I went to System Preferences -> Display -> Arrangement and toggled the Mirror Displays check box.

enter image description here

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A self "work around": one way to get the monitor back "alive" is by disconnecting the HDMI cable from my HDMI/USB-C adapter.

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