I’m in the midst of migrating from my old iMac, with its Thunderbolt 1 ports, to my new iMac Pro with Thunderbolt 3. A key purchase was an Apple Thunderbolt 3 to 2 adaptor, which lets me hook my Thunderbolt 3 external drives up to my old iMac. Well, it would if it worked.
When you buy Apple’s adaptor, its box shows it being used to do just that: connect a Thunderbolt 1 external drive to a Thunderbolt 3 port. Look very carefully at that diagram, though, and you might notice that the external drive is a four-disk array. Not shown there is its mains power supply, without which the adaptor doesn’t work. Apple’s little secret – mentioned at the very end of this support note – is that this adaptor doesn’t pass power through it.
There are, thankfully, several options with which you can work around this surprising limitation. In my case, I also have a USB 3 housing for that SSD, so I have just popped it out from its normal Thunderbolt 3 case into that. You may instead be able to connect a Thunderbolt 3 case to a USB 3 port using a USB-C to USB 3 adaptor, although I haven’t tried that. One Thunderbolt 1 hard drive that I have sports its own USB 3 port too, so that won’t be a problem with my new iMac Pro.
This doesn’t affect larger devices, such as my Promise Pegasus RAID array, as they don’t draw power over their Thunderbolt connection. Once that is hooked up to my iMac Pro using Apple’s adaptor, that will also provide a useful Thunderbolt 1 port, for backward compatibility.
So when you’re planning to mix Thunderbolt 3 with 1 or 2, bear in mind that it may not prove as simple as the diagram shows.