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Battery-powered 2018 MacBook Air will only negotiate Thunderbolt for first connected device  

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I've found an interesting thing: when one device (of any kind) is plugged into a USB-C/ThunderBolt 3 port on my 2018 MacBook Air while on battery power, the second port will only let things connect as USB. 

A few scenarios to illustrate this:

  • Thunderbolt 3 Hard Drive plugged in = another Thunderbolt 3 hard drive won't mount until first one is disconnected.
  • USB 3.x dock/hub plugged into Port 1 = Thunderbolt 3 hard drive won't mount when plugged into Port 2 until Dock is disconnected.
  • USB 3.x Dock/Hub plugged into Port 1 = Hard drive capable of both TB3/USB3 will negotiate a USB3 connection.
  • BUT... if Thunderbolt 3 Drive is plugged in first, then that same USB 3.x dock/hub will plugin and mount correctly. 

If the MacBook Air is powered – say, via a Power-Delivery-Capable TB3 or USB Dock – then both ports will negotiate a Thunderbolt connection regardless of what else is plugged in.

I've confirmed this on two different 2018 MacBook Airs, so I know it's not some anomaly with one.

It seems, perhaps for power consumption purposes, Apple has configured the 2018 MacBook Air to only negotiate a Thunderbolt 3 connection for the first device plugged in when on battery power. Maybe?

My question: do Apple's other Thunderbolt 3 laptops behave the same way? Anyone with, say, a 16" MacBook Pro (or even a 13" MacBook Pro) able to test and report?

This topic was modified 1 month ago 3 times by Dave Hamilton

@davehamilton

Monday, at work, I have some 2 and 3 port TB3 dongles I could try on my 15” 2016 Macbook Pro and the company 13” 2017 Macbook Pro.

2 Answers
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I'm not sure if we are doing the same tests, but here is what I've tried.

Battery only, 2016 15" Macbook Pro w/Touchbar, 4 TB3 ports (1st version with butterfly keyboard).

  • Apple HDMI/USB3/USB-C pass-thru power Thunderbolt 3 dongle.
  • Omars HDMI or VGA, two USB3, Gigabit, pass-thru power TB3 adapter

I was able to drive USB3 and HDMI on each adapter.

pass-thru power was not used.

 

Battery only, 2017 13" Macbook Pro WITH function keys, 2 TB3 ports on left side

  • Omars HDMI or VGA, two USB3, Gigabit, pass-thru power TB3 adapter
  • CableDeconn HDMI or VGA, two USB3, Gigabit, SD card, headphone, pass-thru power TB3 adapter

I was able to drive HDMI on the CableDeconn and VGA on the Omars (did not try any USB3).

I also switched out the Omars for a Tripp-Lite DVI, USB3, Gigabit, Pass-thru power TB3 dongle.

pass-thru power was not used.

 

What I do not know is if my adapters are doing the same things your TB3 adapters and TB3 devices are doing.  Are we doing Pears vs Aprecots ???

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Thanks, @datafornothinandbitsforfree – I guess the question is: does your Mac see the second device you've added as Thunderbolt or USB (in System Information)?

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