Using an old iMac as a monitor

Rachel, sorry your new MM is too new to work ithe TDM. If you had a 2019 or earlier MM it would work. In short, you need to buy an external display for your new MM, which btw is a nice machine! The bad news is TDM was discontinued over 6 years ago now but the good news is external display that have similar displays to your 2009 can be had in the $130 price range. Look at Costco.com and you will find quite a few.

rfaufmann87 has answered your question in a nutshell. This could only be done with a very few and very specific older iMacs and support was very limited. Read this topic for a 2021 perspective on this: https://discussions.apple.com/thread/252271436?answerId=254300142022#254300142022

and this 2021 post by Allan Jones about software limitations:

https://discussions.apple.com/thread/252344497?answerId=254428494022#254428494022

and read about Screen sharing https://support.apple.com/guide/mac-help/share-the-screen-of-another-mac-mh14066/mac

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

The part about 2019 or earler is for Thunderbolt iMacs. The 2009 iMac uses Mini DisplayPort. You need a USB-C to Mini DisplayPort cable, such as:

mophie USB-C Cable with Mini DisplayPort Connector - Apple

Malcolm J. Rayfield wrote:

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

In this case where we are dealing with a 2020 M1 Mac this is the key sentence from that article, "You can connect any of these older iMac models to another Mac from 2019 or earlier for use as an external display." That's what the "too new" in the first response was referring to.

See:

https://discussions.apple.com/thread/251704486

It has a 2020 Mac using a 27" 2009 iMac as a display. There are posts in that thread about failures, but they were using newer iMacs with Thunderbolt, not 2009 or 2010 iMacs with Mini DisplayPort.

When a 2009 or 2010 27" iMac is in Target Display Mode it acts just as a Mini DisplayPort monitor, and will work with any computer that supports Mini DisplayPort, including Windows PCs.

Newer (2011 to 2014 using Thunderbolt) iMacs in Target Display Mode apparently do not appear exactly as Thunderbolt displays and the other Mac has to do something special to use them as displays. 2020 Macs can't do this, but Apple's article on Target Display Mode doesn't distinguish between Thunderbolt and Mini DisplayPort when it say the source Mac can't be newer than 2019. The article is marked as archived, so probably will never be updated since the error is applicable only for computers that are over ten years old.

M1 iMacs simply do not support Target Display Mode, Apple did not explain why and we cannot speculate on their policies on these forums so suffice it to say if your M1 based iMac needs an external display you need to buy one.

One reason they can give for not continuing TDM is the fact that it is not very green. When you use an iMac as a monitor, there is a *lot* of hardware, including a very power hungry CPU that basically does nothing.

No.

Yes, that is an option if you are not in a hurry. I bought a 2020 27" last year, pretty nice machine however I totally understand wanting a M1 iMac too. The big question is when will we see it?

Rachel, you can use third party software that will perform similar to Target Display Mode. Duet Display or Luna Display create this functionality wirelessly.

I hope this helps.

Paul