Thunderbolt 3 (usb-c) to thunderbolt 2 suddenly stopped working...

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Level 1

8 points

Thunderbolt 3 (usb-c) to thunderbolt 2 suddenly stopped working...

I've been using my iMac (2015) as an external display for my MacBook Pro (2019) for the past 7 months with no problems at all, using the Apple Thunderbolt 2 (USB-C) to Thunderbolt 2 adapter.

However, a few days ago totally out of the blue it stopped working, at the same time as my iMac ran a software update. I have since ran software updates on both machines to make sure they are up to date but that didn't solve it.

All I get now is an alert on my MacBook Pro saying "Cannot Use Thunderbolt Accessory Connect accessory using a Thunderbolt-capable cable." when I plug it in. Nothing happens when I press cmnd + F2 on my iMac.

Please help — is this a known issue with the new software update that will be fixed soon? Or something weirder going on...?

iMac 21.5″, macOS 10.15

Posted on Nov 9, 2020 1:31 AM

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Answer 1

Nov 19, 2020 1:00 PM in response to GGrimshaw20

<<All I get now is an alert on my MacBook Pro saying "Cannot Use Thunderbolt Accessory Connect accessory using a Thunderbolt-capable cable." >>

Your original title said the Thunderbolt-3 <--> ThunderBolt-2 device had failed. Maybe you guessed right originally.

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Answer 2

kaz-k

Level 9

51,108 points

Nov 9, 2020 5:24 AM in response to GGrimshaw20

Hi,

Try to run Apple Diagnostics if it reports something which is/are related USB-C(Thunderbolt 3) and Thunderbolt 2.

https://support.apple.com/en-us/HT202731

Apple Diagnostics Reference of codes.

https://support.apple.com/en-us/HT203747

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Answer 3

GGrimshaw20 Author

Level 1

8 points

Nov 19, 2020 6:20 AM in response to Grant Bennet-Alder

Hey Grant, thanks for your response.

I have cleared my iMac and reverted back to Yosemite, but unfortunately it hasn't fixed the issue.

I tried it with my personal MacBook Pro (2019, running Big Sur) and my work MacBook Pro (2019, running Catalina), but target display doesn't work with either anymore.

It was working fine with both running Catalina previously.

Any other ideas?

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Answer 4

Nov 9, 2020 7:00 AM in response to GGrimshaw20

Your iMac can not run any major version of MacOS NEWER than High Sierra, or the feature is not supported.

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Thunderbolt 3 (usb-c) to thunderbolt 2 suddenly stopped working...

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