How to Fix Dual Monitors Not Working in macOS High Sierra?

Extra screen space can make your work more efficient. You don’t have to limit self to just your Mac; instead, you can extend the screen space by connecting one or two additional displays, and enjoy the benefits of multitasking.

Setting up an external screen(s) is not a complicated task; you just have to follow step-by-step instruction. However, some users may still experience certain issues. This especially applies to Macs running on High Sierra.

In this guide, we give a simple instruction on connecting an additional screen to your Mac. We also provide a list of problems you might experience when arranging Dual Monitors on macOS High Sierra. Find out the fixes to these problems below.

Connecting an external monitor to your Mac

With an additional display connected to your Mac, you can set up:

  • An Extended Desktop, to let one display become an extension of the other.
  • Video Mirroring, to let an additional display(s) show the same windows and content as the main/dominant display.
  • Closed-display mode, to be able to use the external display while your Mac’s lid is closed.

Follow this step-by-step instruction to attach an external monitor to your Mac.

  1. First of all, you should find out which type of cable and Apple adapter you need to get the monitor and your Mac connected.
  2. Plug the adapter/cable in.
  3. Go to System Preferences, open the tab Displays.
  4. Click the tab Arrangement.
  5. Make sure to NOT check the Mirror Displays in (unless you’d like the second screen be showing exactly the same content as the original one).
  6. Choose the illustrated displays and drag them to the arrangement you need. On the lead screen, you will see a white menu pop up (starting with OS X Mavericks all screen have their own menu bar). Drag the white menu and place it on the screen you want to appoint as dominant.

To have the displays mirrored on both/multiple displays, open System Preference, go to Displays – Arrangement and choose Mirror Displays.

You can display the second screen on your Apple TV using AirPlay.

Fix: Connecting dual monitors in High Sierra

Having upgraded to macOS High Sierra, many users have reported facing certain issues with connecting the external monitors to their Mac. In particular, many could get only one extended screen be synced with Mac, while the other stayed fully or partially irresponsive.

A Mac fix prompt would appear on the screen suggesting them to reset SMC. This would help to fix the problem, but only temporarily. With the next reboot, they couldn’t get the second screen connected again.

In such a case, the best advice would be to start with upgrading to 10.13.1 High Sierra version. Next, you should check the versions of the display ports on the displays and reset the monitors/displays themselves, as well. You can also check the cables to have no physical damages. In case you don’t want to install the recent upgrade, you can clear the caches and plists. Some of these actions should definitely help you fix the problem.

Other fixes you can try:

  • Get the drivers to your external device(s) upgraded. Updating to fresh firmware may help you get the problem solved.
  • Install the DisplayLink Drivers, to fix the possible sound playing issues on your TV.
  • Before you insert the connector into your Mac computer, make sure to open the USB port on the connector. The computer will not recognize the external monitor if a USB port has a cord attached to it.
  • If the problem is still there, even after the OS upgrades to 10.13.2 and 10.13.3 versions, try resetting NVRAM every time after you reboot.

We hope that our guide helped you to fix the problem of dual monitors not working properly on your Mac.

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