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Thunderbolt Mac: Bus-Power Limitation

Created on: August 21, 2023
Last updated: August 21, 2023

Select Thunderbolt 4 & Thunderbolt 3 Mac systems may be subject to a bus power limitation where only two devices at a time are recognized and their volumes mounted. This will depend on the amount of power they draw and/or other factors related to the computer's architecture. This should not impact simple devices like USB-C thumb drives or other devices that draw very little power. 

Which Products Were Tested?

The main products tested to date were the Thunderbolt 3 Envoy Pro EX and the USB-C Envoy Pro EX. Additionally other bus-powered products like the Thunderbolt Envoy Express have exhibited this issue in the field. Other bus-powered devices like the OWC Travel Dock E may also be affected but have not yet been tested in combination with other bus powered products.

Which Macs Were Tested?

We confirmed this issue applies to both the iMac Pro, (likely) all MacBook Pros with Thunderbolt 3, M2 Pro Mac Mini with Thunderbolt 4, and 2018 Mac Mini. For the iMac Pro it was noted that the combination of ports used (two outer, two inner, two on either side) did not seem to matter when tested with the Envoy Pro EX. When connected, the third device was not recognized, but once of the other two were unmounted and disconnected, the third device was recognized and mounted. That said, as a precaution we believe it makes sense to split the two connected devices up based on the bus locations. Ensure one drive is connected to a Bus 1 port and the other the Bus 0 port.

Which Macs Were Not Affected?

The only Mac tested which we confirmed could bus power and mount four separate TB3 or USB-C EX model SSDs at the same time, was the 2019 Mac Pro.

Why Does this Happen?

We believe the most likely explanation is that the Thunderbolt 4 and Thunderbolt 3 ports on a given host may not always be able to deliver 15W each. More likely is that there are 15W available per controller (based on test results so far). While we did not find direct mention of per controller or per port power wattage from either Intel or Apple, there are public documents from each that imply 15W per port is not always possible, even though this is what the promotional language seems to suggest. See the quotes below for more context.

Intel: Thunderbolt 4 and Thunderbolt 3 also guarantees the ability of the host system to send 15W of power on at least one port if a Thunderbolt 3 device that requires bus power is connected.

Apple: If your Mac has multiple Thunderbolt 3 ports, connect your device to a Thunderbolt 4 or Thunderbolt 3 port farthest away from the current port. For example, if your device is connected to the ports on the left side of MacBook Pro, connect to the Thunderbolt 3 ports on the right side.