Can I use an old iMac as a second monitor?

have a similar situation with an iMac 27" running HighSierra 10.13.6 and I'm about to purchase a new Macbook Air (or Pro) M3. I too hate to discard this iMAc as it still functions and the screen is still fantastic. I have an SATECHI USB-C Multiport adapter withUSB-A, USB-C chargeport and 4K HDMI ports (also aSD/Micro slots).

Hi

I believe that Target Display Mode will work only on iMacs from 2014 or before. It used a particular version of a Thunderbolt Display Port, not present on newer models.

It is a real shame that Apple didn’t reintroduce this as they moved forward, especially if you have a beautiful 5K monitor!

I’m looking at converting old dead 27” iMacs with working 5k displays into standalone monitors. It’s not cheap - about £250 for the right graphics adapter - and it’s fiddly but cheaper than a new 5k monitor. Message me for details.

Yes, you can use your old iMac as a second monitor by using Target Display Mode. However, Target Display Mode is only supported on certain iMac models, and unfortunately, your iMac (Retina 5K, 27-inch, Late 2014) does not support this feature.

Alternatively, you can explore third-party software solutions or hardware adapters that allow you to use your iMac as a secondary display. These solutions may not offer the same level of performance or functionality as Target Display Mode, but they can still provide a way to repurpose your iMac's display.

Keep in mind that using your iMac as a second monitor may require additional cables or adapters, depending on the ports available on both your iMac and the computer you want to connect it to. Additionally, you may need to adjust display settings on both devices to ensure compatibility and optimal performance.

You could look into the USB-C version of Luna Display (Astropad). That might let you reuse your 27" 5K iMac as a second display for another Mac.

My understanding is that the Luna Display "dongle" needs a USB-C (DisplayPort) signal, and that the vendor does not support using it with hubs and docks. So you would have to plug it directly into a Thunderbolt port, consuming that port. Then you would want to run another cable between the Macs to help support the video data transfer. (If you were looking at Mac minis, that would be one reason to go for the M2 Pro version with the extra TB ports.)

Even the Astropad site admits that the product has limitations compared to a "hardware" monitor connection.

"Like any product, Luna Display does have limitations and it’s not recommended for high motion content. For example I wouldn’t recommend using Luna Display for gaming or video editing."

https://astropad.com/blog/step-by-step-guide-how-to-use-your-imac-as-a-monitor/

den.thed wrote:

Only if the 2014 iMac is down-graded to macOS High Sierra and the other Mac is a 2019 or earlier running macOS Catalina or earlier.

The OP says they have an iMac (Retina 5K, 27-inch, Late 2014) – one of the first iMacs with a Retina display. It can't act as a Target Display, even if downgraded to High Sierra or below.

please carefully read > Use your iMac as a display with target display mode - Apple Support

Simply put, no Retina iMac's or new Silicone Mac's supports using an iMac in Target Display Mode.

Apple Silicon Macs do not support using any iMacs in Target Display Mode. A Retina iMac released in 2019 or earlier, running Catalina or earlier, can use another old iMac as a Target Display.

Only if the 2014 iMac is down-graded to macOS High Sierra and the other Mac is a 2019 or earlier running macOS Catalina or earlier.

please carefully read > Use your iMac as a display with target display mode - Apple Support

Simply put, no Retina iMac's or new Silicone Mac's supports using an iMac in Target Display Mode.

The third party software that I use is Luna Display. Target display mode isn't possible as den.thed suggested.

Hope this helps.

Paul