Why do I see "Cannot Use Thunderbolt Accessory"?

I purchased a brand new MBP and a cable was included in the box, it has USB-C type connectors on each end and works for charging the computer.

When, instead of charging with the brick, I plug it into my new iMac, I get this error message:

Why do I see this? How do I fix the problem?

This message implies that I have connected the "accessory" (iMac) using a non-Thunderbolt port on the MacBook Pro. However, the marketing materials on apple.com tell me that all the ports are Thunderbolt ports:

Solution 1:

To slow charge your Mac Book Pro from your iMac, you can connect the USB-C Cable. Unfortunately It seems with recent updates is shows this error on both the MBP and iMac screen. However despite this happening, charging is still occurring.

Solution 2:

Why do I see this?

You see this because the computer recognizes a Thunderbolt device was connected but the cable you used doesn't have the "superspeed" wires to support Thunderbolt. The cables that come with Apple laptops are cables with the USB-PD power lines, USB 2.0 lines, but no "superspeed" lines for USB 3.x, Thunderbolt, or other USB-C alternate modes. USB-C uses a USB 2.0 "billboard device" protocol to help with alternate mode negotiation. The computer will see that a Thunderbolt device is connected but is unable to establish a any Thunderbolt communications as the needed wires are not there.

How do I fix the problem?

Use a Thunderbolt cable, not the USB 2.0 "charging" cable that came with your computer.

The error is worded poorly. It may have made sense in the days of Thunderbolt 2 where people might plug in a drive or display using USB for power and Thunderbolt for data. It's saying that it sees something that is advertising itself as a Thunderbolt device on USB, and it's assuming that there's a separate cable for Thunderbolt. With Thunderbolt 3 and USB-C there's only one port needed for power, USB, and Thunderbolt.

Before you go out to buy a new cable see if you have a USB 3.0 USB-C cable in a drawer somewhere, or you can "borrow" from a hard drive or something. A USB 3.0 cable with USB-C connectors on both ends should work as a Thunderbolt cable, but perhaps at lower speeds. If that gets you a Thunderbolt connection then maybe you can save yourself from buying a new cable.

Passive Thunderbolt cables will be short, under 1 meter long, but they can be inexpensive. You might be able to find a Thunderbolt 3 cable for less than $10. A very short cable. If you need a 2 meter cable like the USB-C charging cable that came with your Mac then expect to pay in the $40 to $80 range.