michael saggese wrote:
I have a late 2012 27" iMac that I would like to use as a monitor.
What do you want to use it as a monitor for?
That iMac supports Target Display Mode, but the requirements are very restrictive.
- It must be running High Sierra or earlier. Your plan to "update back to Catalina" is likely to fail, as it sounds like Apple removed Target Display Mode support from Mojave and Catalina.
- The video source (now) must be another Mac with a hardware model year of 2019 or earlier. That Mac must be running Catalina or earlier. This means that no current or recent Macs support using that iMac as a monitor.
- The connection must be a Thunderbolt one. At a minimum, you would need a now-hard-to-find Thunderbolt 1/2 cable. These have Mini DisplayPort connectors on the ends, but are higher-spec. If Apple supported using that iMac as a monitor for modern Macs (which they don't), you would also need to buy an $50 Apple Thunderbolt 3-to-2 adapter (since Thunderbolt 3 and later live on USB-C).
It is likely that you cannot use that iMac as a monitor for your intended application, or that, if you can, that the cost and inconvenience would be prohibitive relative to what real 27" 2.5K and 27" 4K monitors now cost.
As far as getting back to High Sierra, Mojave, or Catalina, if you do not have a bootable external drive (like a clone backup), you may need to install El Capitan or Sierra first. Their installers are available packaged within .DMG files that you could download on another computer, transfer to the old Mac, and then open using Mountain Lion.
