Why can't M1 Max MacBook Pro 16" (2021) use 3 bus powered Thunderbolt 3 devices?

Question

Why can't M1 Max MacBook Pro 16" (2021) use 3 bus powered Thunderbolt 3 devices?

Given that the M1 Max MacBook Pro 16" (2021) is advertised as having 3 independent, full, Thunderbolt 4 ports on their own buses (as opposed to 3 TB4 ports shared across only two TB4 buses on the M1 Pro), why is it not possible to connect three Thunderbolt 3 4TB SSDs to the computer? Two is fine, but add a third and the computer says there is not enough power to connect the third as a Thunderbolt device, so it falls back to USB 3.1. I've verified with many different certified TB3 and TB4 cables, and using 4TB bus powered Thunderbolt 3 external SSDs from Sabrent, SanDisk and OWC. All drives require the 15W (5v 1.5A to 3A) that is provided for on a Thunderbolt 4 bus and port. Yes I can plug in one or more of the drives into a powered TB4 dock/hub, but often I don't want to carry yet another device just to compensate for something that seems like shouldn't be a problem.

Is there any documentation about maximum a power budget across at three Thunderbolt 4 buses/ports combined? I can't seem to find anything like that. All I see ( so far ) is that this model is supposed to have 3 independent Thunderbolt 4 buses with one Thunderbolt 4 port per bus, each bus capable of 15W (5v 3A).

Thanks for any links to some documentation on this matter! 🙂

MacBook Pro (2021)

Posted on Nov 27, 2024 3:10 PM

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Answer 1

Top-ranking reply

Brock Gunter-Smith Author

Level 1

45 points

Nov 29, 2024 8:44 AM in response to Brock Gunter-Smith

SOLUTION: Apple Support says that the M1 Max MacBook Pro does not actually support using a bus powered SSD on each of the 3 independent Thunderbolt 4 ports. Despite product documentation giving the impression that each of the ports does operate on its own Thunderbolt 4 bus (contrary to the M1 Pro MBP that only has two TB4 buses where two of the ports share one bus) it appears that there is some undocumented restriction in the MacBookPro18,2 that prohibits all three ports operating in Thunderbolt 4 mode at the same time when the devices connected are drawing power at the "allowed" 15W (5V 1.5-3A) level.

So really at the end of the day you can only use two TB4 devices that draw 15W (5V 1.5-3A) at the same time, which will reduce the allowed power draw on the third "Thunderbolt 4" port to be 500mA operating in USB 3.1 mode. There doesn't appear to be any public Apple supplied statement about this limitation, which is a tad disappointing because while you certainly can use Thunderbolt 3/4 devices on all 3 ports simultaneously, they cannot all be bus powered.

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Answer 2

Allan Jones

Level 9

79,802 points

Nov 29, 2024 10:46 AM in response to Brock Gunter-Smith

Thank you, Brock. That's good follow-up info. If you wish to place your justifiable concern regarding the documentation before an Apple employee, you can use this feedback link:

Why can't M1 Max MacBook Pro 16" (2021) use 3 bus powered Thunderbolt 3 devices?

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