Issues using my 2013 27 inch iMac as an external Monitor with my 2020 27inch iMac.

The 2020 iMac does not support Target Display Mode with a Thunderbolt iMac.

Use your iMac as a display with target display mode - Apple Support

"You can connect any of these older iMac models to another Mac from 2019 or earlier for use as an external display."

• 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.

More info here: https://support.apple.com/en-us/HT204592

Target display mode was removed as a feature in the Late 2014 5K iMac, apparently due to the fact that a single Thunderbolt 2 cable could not drive the full resolution of the new ultra high resolution display. Every later Mac after came without the feature.

[Edited by Moderator]

jamierosevideo wrote:

Thanks for the info. Do you know why the 2019 iMac supports TDM but the 2020 doesn't? Or is it simply because apple want you to buy their displays?

The 2020 27" iMac have had a fairly significant hardware change which is likely

the reason. Besides, Apple no longer markets "their own displays".