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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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5 Answers 5

Reset to default
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Latest workaround:

  1. Wake your mac. If all monitors are "online", skip to last step (step 4).
  2. Once you see the login screen in one of the monitors, press 'Esc' to lock/"sleep" it again. Wait for 2 secs.
  3. Now wake the mac by pressing any key. Wait for 5-10 secs. [All the monitors should be online by now]
  4. Proceed with login

Earlier workaround:

This one worked, too ... but is obviously more of a hassle: re-seat (disconnect + reconnect) the HDMI cable from my HDMI/USB-C adapter.

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    Still facing this issue. Any other workaround?
    – Mahendran
    Commented Aug 8, 2019 at 6:06
  • Thanks for sharing. It really worked for me. But I had to wait for 4-5 seconds before the second monitor come online. Then I typed my password. Let's add this to answer.
    – Mahendran
    Commented Aug 8, 2019 at 11:20
  • @mahemadhi At least for me, I dont have to "wait" for the monitor to become online. After ESC, wait a bit ... then start by pressing a key like "shift" to wake up ... and then simply type your password and hit ENTER. At some point, both monitors will be "up", and you are already logged in. Meaning: you dont have to wait for the login panel to become visible to enter the password. Downside: if you got it wrong, you are logged in, but one monitor stays black. Then you have to explicitly log out (ctrl cmd q), type ESC, wait ...
    – GhostCat
    Commented Aug 8, 2019 at 11:35
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When I faced this issue, I pulled the StarTech Adaptors in and out multiple times and the issue would resolve itself. Now I found a better solution as described below.

I went to System Preferences -> Display -> Arrangement and toggled the Mirror Displays checkbox (ie check and then un-check it again).

enter image description here

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  • This worked for me too, woohoo!
    – pfrank
    Commented Mar 27, 2020 at 15:06
  • This shouldn't be necessary, but it seems to work
    – Rob
    Commented Feb 20, 2021 at 0:34
  • Doesn't work. and the panel with mirror checkbox is in the black monitor.
    – towry
    Commented May 7, 2021 at 1:05
  • Quite scary if you accidentally mirror your primary black screen onto the still working screen! In the Displays tab > arrange... > I dragged my black laptop screen onto the working LCD I plug into to mirror, which gave me all black. Physically unplugging and replugging in the cable did not help. Was about to give up and hard reboot (ugh), but after locking the screen by dragging to a hot corner and waking it back up again, I was lucky enough to notice a cursor appear on black, and finally the mirror kick in.
    – qix
    Commented Mar 27, 2023 at 6:05
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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.

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    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
    Commented Oct 10, 2017 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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If the lock screen is on after logging in, press ^ Control⌘ CommandQ to lock it again. Then click "Cancel" at the bottom of the screen, so your Mac display also turns black.

Then unlock it (TouchID while the screen is black does the job, but you can also wake it any other way) and see if the monitor comes up again. In almost all cases, it worked instantly after that workaround.

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