Late 2009 iMac as external monitor on 2020 21.5 inch iMac

Hello, ObsoleteWorkingStill.

Unfortunately, that setup is not supported by Apple. The iMac cannot be used as a monitor for any of the newest Macs and target display mode is no longer a feature of macOS.

As Apple states them, here are the rules for target display mode on iMac:

click> https://support.apple.com/en-us/HT204592

Per the link above:

• Make sure that your target iMac is using macOS High Sierra 10.13.6 or earlier. 

• The source Mac must have been introduced in 2019 or earlier and have macOS Catalina or earlier installed.

D.I is correct. There are two 3rd Party Apps that will allow you to use an imac as a second monitor. DuetDisplay and Luna Display.

I'm using Duet Display on my 2012 27" imac. I'm not sure what the minimum OS requirements are.

Paul

ObsoleteWorkingStill, yes sort of. 2009 iMac will still need to have the system installed but you can use the remaining disk space as an external drive. You can setup file sharing on the 2009 iMac and then access the drive via the finder. Look at the sharing system preference on the 2009 iMac and enable the directory that you want to share. It will operate similar to inserting a USB drive in your mac. I've done this in the past.

I hope this helps.

Paul