Thunderbolt 3 (USB-C) to USB-C adaptor doesn't work on older Macs?

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Thunderbolt 3 (USB-C) to USB-C adaptor doesn't work on older Macs?

I have acquired a Thunderbolt 3 external disk.

I bought an Apple Thunderbolt 2 cable and Thunderbolt 3 to Thunderbolt 2 adaptor as indicated by LaCie, and connected the disk to my iMac (Late 2015) running Catalina. The disk powers up and then shuts down.

Is this because the adaptor is really only for connecting a new iMac to an old Thunderbolt 2 device? I understood it to work in both directions but my setup does not work. Same problem on a 2015 MacBook.

iMac 27″, macOS 10.15

Posted on May 22, 2021 8:10 AM

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Answer 1

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SeaPapp

Level 7

26,486 points

May 23, 2021 3:40 AM in response to hoquet

This adapter is bidirectional, which means you can use it to connect Thunderbolt 3 devices to a Mac that has a Thunderbolt or Thunderbolt 2 port. In that case, the Mac must be using macOS Sierra or later, and the device using Thunderbolt 3 must provide its own power.

About the Apple Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter - Apple Support

The LaCie 2big Dock DOES use external power and you exceed the software requirements so it should work.

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Answer 2

hoquet Author

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May 23, 2021 3:43 AM in response to SeaPapp

Yhanks for your reply. The disk has its own power and the iMac is running Catalina 10.15.7.

The key text in your link seems to be this : "The Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter connects a Mac that has a Thunderbolt 3 port to a device that uses a Thunderbolt or Thunderbolt 2 cable. "

Whereas my Mac has a TB2 port (built before TB3 ports existed) and the disk port is TB3 (USB 3.1) i.e. the reverse situation.

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Answer 3

hoquet Author

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May 23, 2021 5:56 AM in response to SeaPapp

Right... "This adapter is bidirectional, which means you can use it to connect Thunderbolt 3 devices to a Mac that has a Thunderbolt or Thunderbolt 2 port. In that case, the Mac must be using macOS Sierra or later, and the device using Thunderbolt 3 must provide its own power."

What I'm saying is that this doesn't work. The TB3 disk does not connect to the Mac via the adaptor and TB2 cable. So I guess the TB2 cable is the problem?

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Answer 4

SeaPapp

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May 23, 2021 6:23 AM in response to hoquet

It could be the cable, it could be the adapter. Are both the cable and adapter from Apple or are they third party? The reason I ask is that sometimes third party devices aren't made with the same quality, I found out the hard way with a third party lightning to hdmi adapter where a more expensive one from Apple worked fine.

Also, and don't be offended, make sure you’re connecting to one of the thunderbolt ports on the back of the drive and not the usb3.1 port which looks the same.

You might try resetting the SMC on your iMac How to reset the SMC of your Mac - Apple Support

After that I would contact LaCie support, it should work.

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Answer 5

hoquet Author

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May 23, 2021 1:38 AM in response to den.thed

It is a LaCie 2big Dock Thunderbolt 3 disk. It works fine with the USB-A cable connection, so there is no crisis, but I was expecting to be able to free up a USB slot by using one of the two Thunderbolt 2 slots on the iMac.

After finding the adaptor/slot/cable didn't work, I searched in Apple documentation and have not found anything to confirm that this setup will actually work. But LaCie says :

I've come to the conclusion that the adaptor is not intended for this purpose. Or my 2015 iMac is too old.

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Answer 6

den.thed

Level 10

82,636 points

May 22, 2021 8:22 AM in response to hoquet

What Thunderbolt 3 external disk?

Pretty sure that it will only work, if the TB 3 external disk is self-powered.

FWIW if the LaCie external drive also provides a USB-A 3.0 cable connection, then that would be the fastest and easiest connection to use with your 2015 Mac’s.

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Answer 7

hoquet Author

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May 23, 2021 6:43 AM in response to SeaPapp

Both cable and adaptor are new from Apple. I never buy third party stuff that may not work, despite the temping prices.

Not offended! The whole USB/TB thing is so confusing now. Everything used to just work on Apple.

There are two ports on the back and I believe the one next to the USB-A ports is definitely TB2. I will try again next time I shut the iMac down and will reset the SMC (done fairly often because the Mac has been crashing and restarting by itself of late). Many thanks for your help.

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Answer 8

SeaPapp

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26,486 points

May 23, 2021 9:58 AM in response to hoquet

On a completely different topic, if your iMac is having kernel panics when it sleeps stop putting it or letting it go to sleep. Catalina and Big Sur have problems with sleep. Instead just pit your display to sleep and the panics will probably stop.

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Answer 9

tbirdvet

Level 6

14,567 points

May 22, 2021 11:16 AM in response to hoquet

Most if not all TB3 drives (or connections) will not work using TB3 to TB2 adapter.

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Answer 10

hoquet Author

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May 23, 2021 1:49 AM in response to tbirdvet

OK I understand. TB3 is new technology that is making 'old' kit redundant.

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Answer 11

SeaPapp

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May 23, 2021 3:49 AM in response to hoquet

Read past the first sentence of the article I linked. Read the data transfer section which includes the text that I copied and posted in bold.

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Answer 12

den.thed

Level 10

82,636 points

May 23, 2021 11:34 AM in response to hoquet

It’s not going to be or run any faster on TB 2. So I would run it on USB-A and use a self-powered USB-A 3.0 hub to expand the number of USB-A 3.0 ports.

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