Level 1 (4 points)

Why is my second Thunderbolt display not working after Sequoia 15.8 update?

I've been running two Thunderbolt displays off a 2018 Mac Mini 3.2GHZ i7 with minimal difficulty for years. After upgrading to Sequoia 15.8 my Mac can't detect the second display and the primary display blacks out for no reason. Unplugging and replugging the first display lights it up again, but the second display remains dead. I've unplugged, rebooted, trashed the windowserver plist and whatever else I can think of to no avail. Can anyone help me here? What am I missing? Thanks

Posted on Sep 22, 2026 6:39 AM

5 replies

Grant Bennet-Alder

Level 10 (148,617 points)

Sep 22, 2026 7:53 AM in response to Kale Browne

first, can you confirm which displays you are using?

There are TWO displays, commonly mistaken for each other that appear to have a Thunderbolt-2 interface.

The Apple Thunderbolt Display 27-in A1407 is a 2560 by 1440 display that has a lead in cord with ONLY TWO connectors -- Thunderbolt-2 and MagSafe.

This display works great with the Apple ThunderBolt-3 <-> Thunderbolt-2 adapter, US$50.

In addition to the built-in Thunderbolt cable, the display also has a built-in MagSafe-2 connector that charges an older Thunderbolt-equipped MacBook Air or MacBook Pro.

from:

https://everymac.com/monitors/apple/thunderbolt/specs/apple-thunderbolt-display-27-inch-specs.html

--------

Cinema display LED 27-in A1316 from 2011 is a 2560 by 1440 display that features lead-in cord with three connectors -- Mini DisplayPort, MagSafe-2 and a USB-A pigtail. It does NOT work with the Thunderbolt adapter, but works fine with a USB-C to miniDisplayPort adapter.

It includes three USB 2.0 ports and a convenient single cable with THREE connectors (Mini DisplayPort, MagSafe-2, and USB 2.0), a built-in iSight camera with a mic and a 2.1 speaker system.

This display works great with a USB-C to MINI-DisplayPort adapter.

from:

https://everymac.com/monitors/apple/studio_cinema/specs/apple-led-cinema-display-27-inch-specs.html

Grant Bennet-Alder

Level 10 (148,617 points)

Sep 22, 2026 12:11 PM in response to Kale Browne

What happens when you connect ONLY the most troublesome display, by itself?

You did not cite any case where that one alone lights up, suggesting a possible display failure.

Grant Bennet-Alder

Level 10 (148,617 points)

Sep 22, 2026 2:46 PM in response to Kale Browne

if you have some reason to believe yours has failed due to problems with the captive lead-in cord’s Thunderbolt-2 connection, Users have successfully connected a discrete Thunderbolt-2 cable to the second Thunderbolt-2 port (on the chassis) and had the display work fine.

Kale Browne Author

Level 1 (4 points)

Sep 23, 2026 4:21 AM in response to Grant Bennet-Alder

Thanks. Good call. It does appear to be a display failure.

Kale Browne Author

Level 1 (4 points)

Sep 22, 2026 11:36 AM in response to Kale Browne

Thanks. Both are A1407 and both are connected with an Apple adapter to the USB-C ports. They worked fine with earlier Sequoia versions, but this last 15.8 update completely tanked access to the second display for some reason.

Originally posted at discussions.apple.com/thread/256362664