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All of a sudden I can't use my old iMac as a second monitor

For well over a year I have been using an old iMac (27-inch, Late 2013) now running macOS 10.15.7, as a second monitor for my MacBook Pro (15-inch, 2019). I have a USB-C to lightening adaptor and I hook up the old mac through one of its two lightning ports. The keystroke on the MacBook to add the old Mac as a second monitor is (Command-F2). This has been working flawlessly.


I recently upgraded my MacBook Pro OS to macOS 11.2.2 and now it will not recognize the monitor connected through the thunderbolt adaptor. Was this function removed fro mac OS Big Sur, or its there now a different keystroke? Is it likely that the adaptor and/or the lightening to lightening cable have failed? Could the thunderbolt jack on my old iMac have suddenly failed (I ran hardware diagnostics [OK] and tried each of the two thunderbolt inputs without success)?


Perplexed in The Peach State...

Posted on Feb 25, 2021 6:23 PM

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Question marked as Apple recommended

The target iMac cannot be running anything later than High Sierra.

The source Mac cannot be from 2020 or later.

And support for this target display mode is disappearing, as was mentioned.

Posted on Feb 25, 2021 7:03 PM

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All of a sudden I can't use my old iMac as a second monitor

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