How can you use an older iMac (2012) as an external monitor with a newer MacBook Pro (2023)?

To use any of these iMac models as an external display:

• The iMac used as a display must have macOS High Sierra or earlier installed.
• The other Mac that you're connecting it to must have been introduced in 2019 or earlier and have macOS Catalina or earlier installed.
• The cable connecting the two Mac computers must be a Thunderbolt  or Thunderbolt 2  cable.

As you can see, this is not working as your new MacBook most likely run a newer macOS then Catalina

Apple seems to have decided this feature is too difficult to support going forward. It also may be too hard to enforce Digital Rights Management on such a display.

Refreshing a modern display is done at the highest data rates of anything your MacBook Pro does. Accessing the drive comes in second. In addition, the timing required is completely unforgiving. This is a very difficult problem, and your Macs were never built for the purpose of accepting display data from an outside source at full-motion Video standards.

The device that IS purpose built for this purpose is your TV set. Or an external display of your choosing.

joschro02 wrote:

I'm not done here. I still don't have a good answer as to why this functionality was taken away…

The answer is “no. This does not work.”

The reason why? Nobody here knows why Apple might have removed this.

Good displays are available less expensively than the Apple Studio displays and Pro Display XDR, as well. There’s a nice top-of-the-line 42” OLED with 4K (2160p) resolution for ~USD$1000, for instance.

The old iMac displays from circa 2012 are 2560 by 1600; not that far past 1080p TV. For comparison, Apple Studio display is 5120 by 2880. (If you do want a lower-resolution non-Retina display such as a 1080p TV display, also look for info on enabling AppleFontSmoothing from the command line, too.)

Buy what works for your needs and your budget. if that’s not Apple gear, so be it.

joschro02 wrote:

I'm not done here. I still don't have a good answer as to why this functionality was taken away.

Did you contact Apple for an answer, as I suggested?

That's when maybe they'll realize this whole planned obsolescence thing was not the best idea.

Was that their response?

Apple took away Target Display Mode on the iMacs themselves when the first 27" 5K Retina iMac came out in Late 2014 – long before there was a 27" 5K Studio Display. Probably long before there was a standalone display on the drawing board.

iMacs that supported both TDM and Thunderbolt required Thunderbolt connections to use TDM. That sounds like a technical restriction, not a marketing one. You'd need $80 in Thunderbolt hardware ($50 TB3-to-2 adapter plus $30 TB 1/2 cable) to make a Thunderbolt data connection between a modern TB3/TB4-equipped Mac and a 2012 iMac. To reuse a 27" 2560x1440 or 21.5" 1920x1080 display. Displays have come down in price since Late 2009, when a 27" Dell 2560x1440 monitor went for $1200 ($1000 if they had it on a good sale). Even if you could reuse a 2012 iMac's screen as a display for a Mac Studio, the economics of connecting the two machines might strongly discourage you from doing so.

So the iMacs where the new restrictions on "the other Mac" hurt are the 27" ones from 2009 and 2010, because those Macs only required a Mini DisplayPort connection – not a more exotic and expensive Thunderbolt one.

joschro02 wrote:

I did contact Apple support. They didn't have an answer as to why the function does not work anymore, but they told me to suggest a product change, to bring the functionality back.

So did you do that?

Please refer to Use your iMac as a display with target display mode - Apple Support. Those two Macs do not meet its requirements.

Apple does not publicly discuss their reasons for doing what they do. If you want to contact Apple, Contact Apple.

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