I have an iMac 21.5 inch, late 2009 macOS Catalina version 10.15.7. I would like to use it as a second monitor for my MacBook Air (M2, 2022), macOS Sequoia 15.5. Could you please guide me in the process? I'm not sure, and there is a lot of information that confuses me
[Re-Titled by Moderator]
Original Title: iMac 21.5 inch, late 2009 second screen
MacBook Air, macOS 15.5
You don't.
The 27" Late 2009 iMac supported Target Display Mode using Mini DisplayPort input – but the 21.5" model never did. Apple later added restrictions such that no recent Mac supports using ANY iMac as a Target Display.
You say that you're running Catalina. It is not supported on iMacs released prior to 2012. The 21.5" iMacs which Apple released in Late 2012 through Mid 2014 supported Target Display Mode, but only using Thunderbolt input. You would need to spend $80+ on adapter and cabling hardware to make a Thunderbolt connection from one of them to a modern Mac – only to find that you still couldn't use the iMac as a display for the new machine. (The Thunderbolt connection would be necessary, but not sufficient, to satisfy the conditions for using Target Display Mode.)
You don't.
The 27" Late 2009 iMac supported Target Display Mode using Mini DisplayPort input – but the 21.5" model never did. Apple later added restrictions such that no recent Mac supports using ANY iMac as a Target Display.
You say that you're running Catalina. It is not supported on iMacs released prior to 2012. The 21.5" iMacs which Apple released in Late 2012 through Mid 2014 supported Target Display Mode, but only using Thunderbolt input. You would need to spend $80+ on adapter and cabling hardware to make a Thunderbolt connection from one of them to a modern Mac – only to find that you still couldn't use the iMac as a display for the new machine. (The Thunderbolt connection would be necessary, but not sufficient, to satisfy the conditions for using Target Display Mode.)
Unfortunately that capability was discontinued about 7 or more years ago. The M2 MBA is too new. You can get inexpensive 27" monitors for not much over $100 US or higher quality monitors, up to 32" 4K, for $331 US.
It's time to retire the 2009, give it a rousing Irish wake sendoff and recycle it. If you recycle it be sure to prepare it according to this Apple document: What to do before you sell, give away, or trade in your Mac - Apple Support . Actually you can ignore the step of installing a new system. That's only for selling or giving away.
See the restrictive hardware and OS requirements for the now-deprecated Target Display Mode here:
Use your iMac as a display with target display mode - Apple Support
Sort answer: You don't.
Longer answer: There may be a software based solution, but the lag will be significant. The iMac is not a monitor. You can not plug into it and use it as one.
How do I use my iMac as second screen for my MacBook Air?
