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Level 1

4 points

iMac as external monitor

Hi,

I've bought a second hand iMac (27-inch, Late 2009 - with mini display port), which I hoped to use an external monitor with my MacBook Pro (13-inch, 2017, Four Thunderbolt 3 Ports).

I tried to connect the two devices using a thunderbolt cable paired with a USB-C til Thunderbolt 2 adapter.

When I connect the devices it simply says no device connected. Can someone help me with figuring out whether I'm using the wrong cables? Or perhaps someone has another suggestion to what the problem may be?

MacBook Pro 13″, macOS 10.13

Posted on Sep 5, 2021 11:29 AM

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Level 4

3,445 points

Posted on Sep 5, 2021 11:54 AM

External Display Mode support was dropped with Big Sur.

To make it work:

1) The iMac must be running High Sierra or older

2) The MacBook Pro (or other connecting Mac) must be running Catalina or older.

https://support.apple.com/en-us/HT204592

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Top-ranking reply

Level 4

3,445 points

Sep 5, 2021 11:54 AM in response to Mæh

External Display Mode support was dropped with Big Sur.

To make it work:

1) The iMac must be running High Sierra or older

2) The MacBook Pro (or other connecting Mac) must be running Catalina or older.

https://support.apple.com/en-us/HT204592

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Level 9

72,380 points

Sep 5, 2021 12:01 PM in response to Mæh

I assume the iMac works fine as a stand-alone computer with no display artifacts?

What mac OS is the MacBook Pro running. It must be 10,5 Catalina or older.

If that is not the issue then, yes, your symptoms suggest a cabling issue issue. It may the the ThunderBolt cable. This Apple article:

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

says the 2009 iMac must use a mini-display port cable:

I think "a text diagram" would be:

2017 MacBook Pro ------ USB-c to T2 adapter ------ Mini-display cable ----- 2009 iMac

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iMac as external monitor

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