imac built-in display will not allow use as main display with external display connected

Question

joemulberry Author

Level 1

4 points

imac built-in display will not allow use as main display with external display connected

SYSTEM DESCRIPTION

iMac Retina 5K, 27-inch, 2019

Processor: 3.7 GHz 6-Core Intel Core i5

Graphics: Radeon Pro 580X 8 GB

Memory: 8 GB 2667 MHz DDR4

Startup Disk: Macintosh HD

macOS: Sonoma 14.0

Displays:

built-in: 27” Retina

external: TCL 43S426 - 43" Class 4-Series 4K UHD HDR TV -- connected using Apple USB-C Digital AV Multiport Adapter and HDMI cable

Issue:

I can no longer use my TCL external display as an extended monitor. The system is treating the external monitor as the main display. This problem may have started with upgrading to Sonoma 14.0 – prior to the upgrade everything was working fine (for years). Retina was the Main Display and TCL was the extended.

Built-in Display does not offer the option to use as Main Display -- see panels below showing display settings info for the external and built-in monitor.

Built-in display selected:

external display selected:

If I “Stop Mirroring” the display swaps out of main screen leaving me without access to display settings. If I then Command-F1 to get back to settings, I am effectively back where I started.

iMac (2017 – 2020)

Posted on Nov 1, 2023 8:28 AM

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Answer 1

joemulberry Author

Level 1

4 points

Nov 1, 2023 12:38 PM in response to den.thed

When I click Arrange, the displays are stacked because they are mirrored, and I can't drag the menu bar between them. If I option-drag the top display off to the side, the whole display goes dark and then repaints as the mirror display. To get back to the display showing the control panels I need to cmmd-f1. But the Arrange window still shows them stacked (i.e. mirrored)

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Answer 2

joemulberry Author

Level 1

4 points

Nov 1, 2023 1:31 PM in response to den.thed

If I stop mirroring in the "use as" drop-down, the main desktop display disappears and the view switches to the wallpaper for the external display. Then, when I disconnect the external monitor the main desktop display reappears. And then, when I reconnect the external monitor, the main desktop display disappears again and the view switches back to the wallpaper for the external display.

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