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Howdy,

Target Display Mode does not use Lightning connections.

Which iMac are you using as a 2nd display and what OS is it running?

Not all iMacs can be used in Target Display Mode or as the main computer, and the "display iMac" must be on macOS High Sierra or earlier.

From Use your iMac as a display with target display mode

You can connect any of these older iMac models to another Mac from 2019 or earlier for use as an external display. Learn how to identify your iMac model.

• 27-inch iMac models introduced in 2009 and 2010: Use a Mini DisplayPort cable to connect your iMac to a Mini DisplayPort or Thunderbolt or Thunderbolt 2 port on the other Mac.
• All iMac models introduced in 2011, 2012, 2013, and mid 2014: Use a Thunderbolt cable to connect your iMac to a Thunderbolt or Thunderbolt 2 port on the other Mac. If the other Mac uses Thunderbolt 3, you can use the Apple Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter with your cable.

Target display mode requires macOS High Sierra or earlier on the iMac used as the external display. It does not work with macOS Mojave or later.

FYI,

I connected my iMac (27-inch, Late 2013) running macOS High Sierra 10.13.6 to my

iMac (Retina 5K, 27-inch, Mid 2015) running macOS Big Sur 11.2

using a Thunderbolt 2 cable

and the 2013 iMac worked fine in Target Display Mode as an external display (Command-F2 to switch).

ivan

If your new Mac is from 2020 or later, it does not support using an iMac as a display.