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User profile for user: Kiki75
Kiki75 Author
User level: Level 1
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External Display Connections - New iMac to iMac 2015

Trying to connect my new iMac 2021 (24", M1) to an external display using my old iMac 27" late 2015.

I have an Apple Thunderbolt Cable (with lightning bolt pictures on it) running from old iMac via a Mac Adapter Thunderbolt 3 to Thunderbolt 2.

The adapter is inserted into the Thunderbolt/USB 4 port.

If I insert the adapter end into the UBS 3 ports on new iMac, an error appears 'Cannot use thunderbolt accessory'...

Am I doing something wrong?

iMac (M1, 2021)

Posted on Apr 4, 2023 3:39 PM

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User profile for user: D.I. Johnson
D.I. Johnson
User level: Level 7
22,355 points

Posted on Apr 4, 2023 3:43 PM

Unfortunately, that setup is not supported by Apple. The iMac cannot be used as a monitor for any of the newest Macs and Target Display mode is no longer a feature of macOS, having been deprecated several years ago.


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


Per the link above:

• Your target iMac (monitor) must be using macOS High Sierra 10.13.6 or earlier. 

• In all cases the source Mac that you're connecting FROM must have been introduced in 2019 or earlier and have macOS 10.15 Catalina or earlier installed.


However, you might consider a third-party solution, for example: Luna Display or Duet Display.


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User profile for user: D.I. Johnson
D.I. Johnson
User level: Level 7
22,355 points

Apr 4, 2023 3:43 PM in response to Kiki75

Unfortunately, that setup is not supported by Apple. The iMac cannot be used as a monitor for any of the newest Macs and Target Display mode is no longer a feature of macOS, having been deprecated several years ago.


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


Per the link above:

• Your target iMac (monitor) must be using macOS High Sierra 10.13.6 or earlier. 

• In all cases the source Mac that you're connecting FROM must have been introduced in 2019 or earlier and have macOS 10.15 Catalina or earlier installed.


However, you might consider a third-party solution, for example: Luna Display or Duet Display.


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User profile for user: D.I. Johnson
D.I. Johnson
User level: Level 7
22,355 points

Apr 4, 2023 5:38 PM in response to Kiki75


Kiki75 wrote:
As I will continue to use my new iMac as my primary computer - will I just need to purchase a new external monitor like a LG (etc..) to have dual screens?

You're welcome.


And yes, the iMac will support one external display in addition to its built in monitor.


Video Support

Simultaneously supports full native resolution on the built-in display at 1 billion colors and:

  • One external display with up to 6K resolution at 60Hz


Thunderbolt 3 digital video output

  • Native DisplayPort output over USB-C
  • VGA, HDMI, DVI, and Thunderbolt 2 output supported using adapters (sold separately)


iMac 24-inch - Technical Specifications - Apple


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External Display Connections - New iMac to iMac 2015

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