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I know that Thunderbolt 3 computers are backward-compatible with Thunderbolt 2 devices using an adapter. What I want to know is if the reverse is true - if Thunderbolt 2 computers can connect to devices using the new standard like USB does, albeit with lower speeds. I can't seem to find information around the interwebs regarding this. I'd appreciate if someone would shed some light on this. Thanks in advance.

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I just found something that sheds some light on my question. On the Apple Thunderbolt 3 to Thunderbolt 2 adapter page they mentioned that the adapter is bidirectional and I quote:

The Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter lets you connect Thunderbolt and Thunderbolt 2 devices — external hard drives and Thunderbolt displays, for example — to any of the Thunderbolt 3 (USB-C) ports on your MacBook Pro. As a bidirectional adapter, it can also connect new Thunderbolt 3 devices to a Mac with a Thunderbolt or Thunderbolt 2 port.*

This means that the adapter may be used to connect the new MacBook Pro to TB1/2 devices or an older Mac to a TB3 device. This definitely gives me the answer that I seek about forward-compatibility. However, this may come with some caveats.

  • For one, the page states that forward-compatibility requires macOS Sierra so it may require specific drivers that comes only with the update.
  • Compatibility of this adapter with Windows is still unknown at this point (though I never mentioned that in the question anyway)
  • Speed will be affected by the slowest connection... though that's expected.
  • Other adapters may not support bidirectional connections so it's best to read the spec sheet or ask the manufacturer, if possible.

I will try to get my paws on this adapter and a Thunderbolt 3 device when they arrive in this part of the woods and try them out. I'll then update my answer depending on what I find accordingly.

UPDATE: I found a video on YouTube on how to connect a Thunderbolt 3 capable NAS to a Mac Mini with Thunderbolt 2 and a follow-up video tests the speed of the makeshift connection. This proves that Thunderbolt is forward-compatible, indeed.

There are a few things to note though:

  • The NAS is powered externally. We don't know if it will work with devices that rely on power from the cable itself.
  • No word on Thunderbolt 1 to 3 forward-compatibility but it should work, theoretically.
  • He didn't seem to mention the version of the OS in the Mac Mini. Also, still no idea regarding Windows compatibility.
  • Will USB-C (non-Thunderbolt 3) devices work? I think it will, but we'll need actual tests to prove it.

I will update this answer further once I get my hands on devices that I can test myself.

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  • This does not provide an answer to the question. To critique or request clarification from an author, leave a comment below their post. - From Review
    – DavidPostill ♦
    Nov 7, 2016 at 12:23
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    Given that I was the one who asked the question, I do think that it does. Since the mentioned adapter is bidirectional, that means it can be used connected to a host or device and that answers my forward-compatibility question. However. I think I should edit the answer to make this point clearer.
    – DeVilFisCh
    Nov 7, 2016 at 13:52
  • The reason I say it is not answer is you haven't tested it yet ...
    – DavidPostill ♦
    Nov 7, 2016 at 15:59
  • That may be the case but a commercial product already states bidirectional capabilities in their spec sheet - meaning they tested it already - which means I don't need to do that anymore to prove that it works. That's enough confirmation to me.
    – DeVilFisCh
    Nov 7, 2016 at 16:04
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    @Stunner, sadly, I haven't been able to buy an adapter yet. Also, the only TB device I have is an Intel NUC and the Apple adapter's compatibility with Windows is unknown at this point. I have yet to buy a TB3 device as well as I don't have a PC that supports it. I wouldn't buy a device I can't actually use, after all, especially considering how expensive they can be. I still want to test different and niche scenarios though, so expect an update when I have the devices I need. I have found some new information regarding compatibility, though. I have updated my answer with what I've learned.
    – DeVilFisCh
    Jul 27, 2017 at 12:52
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I bought the Apple Thunderbolt 3 USB-C to Thunderbolt 2 adapter because I wanted something with bi-directional compatibility. Most reviews talk about how to use an older thunderbolt device with a USB-C MacBook but I needed it to work backwards. I have a workstation dock setup for a single Thunderbolt 3 USB-C connection (Kensington SD2400T Nano Dock). Works great on my USB-C MacBook-Pro but I also wanted to use it with my older MacBook Pro, which only has Thunderbolt 2 connections. This adapter and the Apple Thunderbolt 2 to Thunderbolt 2 cable ( 0.5m male to male), did the trick. My workstation now supports all of my MacBooks - awesome !!

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