I have a MacBook Pro (M4) connected to an external monitor via a USB-C to USB-C cable. It was working fine until I upgraded the OS to macOS 26 earlier today. However, the external monitor is no longer detected, even after I restarted the MacBook Pro.
MacBook Pro 14″, macOS 26.0
Macbook Pro M4 Max 128GB RAM, OS 26.0.1. -> Kensington TB4 Docking Station, LG 49W-L59C Monitor (attached via USBC to Docking Station).
With me everything Thunderbolt (I have a Thunderbolt 4 Dock by Kensington that has been the MBP's primary companion since I brought it home) takes random dumps flapping disconnect/connect after upgrading to OS 26. As others say, undesired behavior stops if you reboot with the TB4 dock connected, or if you leave it alone for a while with the lid open and just wait it out for a *very* long time (1/2h or more, every flap seemingly "lasting longer" on until I feel I can finally close the lid). Open or closed lid doesn't matter (though closed lid goes to sleep eventually).
It flaps all my TB peripherals back and forth as well, so it's the connection dock-Macbook, not USBC monitor-dock. It's insane and it was all working perfectly before the upgrade.
Setup works well with other devices including an non-upgraded Macbook Air M2 I also own.
You can claim "now we perfect video with no dropped frames" all you want, but graceful degradation on regular usage really is table steaks expected behavior in 2025 (and that it "eventually fixes itself" with lid open suggests a software bug, not cables). Submitted a bug and everyone who can reproduce should as well.
I write software (but not Kernel stuff). Here's a log around one of the flip-flaps (filtered by IOThunderboltSwitch) in case interested Apple devs see this.
default 16:31:54.565680-0700 kernel 130796819157us IOThunderboltSwitch(0@1)::finalize - terminate device: Kensington TB4 Docking Station (047d:809b:01) (8087:0b26:03)
default 16:31:55.780139-0700 kernel 130798033610us IOThunderboltSwitch(0@0)::processPlugEvent - Thunderbolt HPD packet for rid = 0 route = 0x0 port = 2 plug = 1
default 16:31:55.780402-0700 kernel 130798033873us IOThunderboltSwitch(0@0)::processPlugEvent - Thunderbolt HPD packet for rid = 0 route = 0x0 port = 1 plug = 1
default 16:31:55.895400-0700 kernel 130798148869us IOThunderboltSwitch(0@1)::syncTargetAndNegotiatedWidth - port (0@0:2) - bonding took 0 ms
default 16:31:55.895863-0700 kernel 130798149332us IOThunderboltSwitch(0@0)::processPlugEvent - Thunderbolt HPD packet for rid = 0 route = 0x0 port = 2 plug = 0
default 16:31:55.907162-0700 kernel 130798160632us IOThunderboltSwitch(0@1)::processPlugEvent - Thunderbolt HPD packet for rid = 0 route = 0x1 port = 2 plug = 0
default 16:31:55.960740-0700 kernel 130798214209us IOThunderboltSwitchIntelJHL8440(0@1)::overrideSupportedCLxStates - clx = 0x00000000
default 16:31:55.961412-0700 kernel 130798214881us IOThunderboltSwitch(0@1)::configureCLx - (0x1 -> 0x1) supported = 0x7 common = 0x0 parent = 0x7 child = 0x0 options = 0x0 enable = 1 current = 0x0 target = 0x0 status = 0x00000000
default 16:31:55.993346-0700 kernel 130798246815us IOThunderboltSwitchIntelJHL8440(0@1)::overrideSupportedCLxStates - clx = 0x00000000
default 16:31:55.994040-0700 kernel 130798247510us IOThunderboltSwitch(0@1)::configureCLx - (0x1 -> 0x1) supported = 0x7 common = 0x0 parent = 0x7 child = 0x0 options = 0x0 enable = 1 current = 0x0 target = 0x0 status = 0x00000000
default 16:31:56.004702-0700 kernel Sandbox: ThunderboltAccessoryUpdaterServi(1256) allow iokit-get-properties iokit-class:IOThunderboltSwitchType7 property:Router ID
default 16:31:56.144109-0700 kernel 130798397578us IOThunderboltSwitchIntelJHL8440(0@1)::overrideSupportedCLxStates - clx = 0x00000000
default 16:31:56.144807-0700 kernel 130798398275us IOThunderboltSwitch(0@1)::configureCLx - (0x1 -> 0x1) supported = 0x7 common = 0x0 parent = 0x7 child = 0x0 options = 0x0 enable = 1 current = 0x0 target = 0x0 status = 0x00000000
default 16:31:56.594343-0700 kernel 130798847808us IOThunderboltSwitch(0@1)::processPlugEvent - Thunderbolt HPD packet for rid = 0 route = 0x1 port = 13 plug = 1
default 16:31:56.691675-0700 kernel 130798945140us IOThunderboltSwitchIntelJHL8440(0@1)::overrideSupportedCLxStates - clx = 0x00000000
default 16:31:56.692414-0700 kernel 130798945879us IOThunderboltSwitch(0@1)::configureCLx - (0x1 -> 0x1) supported = 0x7 common = 0x0 parent = 0x7 child = 0x0 options = 0x0 enable = 1 current = 0x0 target = 0x0 status = 0x00000000
While cable changes may work for others, it hasn't been the case for me. My cables are new T5/80Gb cables (I already paid for a maxed out M4 Max, what's a TB cable?)
What sucks is that every other laptop including my Linux laptops of a huge range of versions and generations all work and behave beautifully with this exact setup... but what's by far the most expensive machine I've ever bought, decides to take random dumps now. It's just frustrating and makes me feel like I wasted my money here (when I know a crappy bus implementation when I use one; I mean, I have windows machines too).
I really hope they fix this. Soon. It's not okay since it hits me at least 2x/day as I move around with my laptop as one does.
Good luck everyone, hope any of this helps.
<< I have a WD19TB and a WD22TB4 dock. >>
Both of those Dell docks claim support for Windows 10, Window, 11, Ubuntu, and ChromeOS. There is no mention that they intended them to work with a Mac of any description.
The checklists for both of those Dell docks suggest that if you have a problem you should update the Dock firmware. Support for that firmware update process is only provided for Windows.
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I think your recognition issue could be caused by lack of generality in the Dell firmware. "Works with Windows, that's good enough" rather than "Works with ThunderBolt across all makers of ThunderBolt hosts."
Same issue here. I can’t use my monitor at work anymore. I have a MacBook M3 Max and a Samsung 34-inch ultrawide monitor. If I plug in on the left side (which I have always used) I only get “share screen” option which gives me 1920x1080 and 30hz. And on the right side I get static flashing image with “snow” on the screen. I reported this to Apple Support on October the 7th, still no update.
[Edited by Moderator]
I have a MacBook Pro which has 2 external usb-c monitors attached through a dock. Although all 3 screens work (including MacBook built in screen) I have noticed a change in behaviour. Prior to MacOS 26.x when I booted up all screens would wake up and show an image. Now with 26.x when I boot up only 1 monitor shows an image (with Logon prompt) and only after I have logged in do the other screens wake up. I use the "Extended" display mode rather than mirror.
<< 2 external usb-c monitors attached through a dock. >>
The Mac does Not support two fully hardware-accelerated displays on ONE USB cable. There is not enough bandwidth to do that in an appropriately-responsive way, so Apple has chosen NOT to support that. Only Thunderbolt docks have enough bandwidth to support two displays, under certain conditions.
That suggests the second display on that dock (or possibly all displays on that dock) are "fake" displays using DisplayLink software and hardware or equivalent, such as J5create.
As such it is no surprise they come up much later.
Just became an issue for me when I upgraded to macOS Tahoe 26.1. On a MacBookPro 14", Nov 2024 (M4,24gb). Seems like a sporadic power cycle. Problem gets worse the more applications the computer is running. This was not an issue at all before this upgrade.
✅ Thank you! The second solution you wrote up (Deep System Reset) worked perfectly.
I've replied and upvoted to flag to others that this works post macOS 26.1 upgrade.
MBP M1 - HDMI to USB-C cable (connected via UGREEN USBC converter with Ethernet cable port and 3xUSB 3.0 ports) - HD monitor (KTC)♥️
I have a MacBook Air M1 and a Mac Mini M4, both of which were connecting to my external LCD monitors wirelessly. Both worked flawlessly until I updated to 26.1. Now, neither display is visible to the Macs.
There are no, and never were, any cables involved in connecting.
Has Apple yet acknowledged they have a problem and WHEN they might have a solution, ready to roll out,
pajacobs--
To get the attention you deserve, PLEASE start a NEW discussion.
If you did not immediately find your solution by reading through the existing postings, you likely do not have "exactly the same problem" and need to explain from the beginning. This discussion is already way too crowded with different problems posted by different users with different hardware, different versions of MacOS, and different displays.
Starting a new discussion will allow Readers to think hard about your exact situation and address the exact circumstances you are facing.
https://discussions.apple.com/post/question
It's M4 Pro. The monitor is a Samsung 34" (CH890), connected via a USB-C to USB-C cable, no adapter or dock.
The external monitor was detected and working fine before upgrading to macOS 26.
<< there are reports online that my monitor model can have issues with the USB-C port on the back. >>
what make&model display is that?
I don't understand your line of questioning. How does this information help resolve the problem - the user says that the monitor worked before MacOs
I am facing the issue with HDMI monitors as well.
Thanks for the tip. Following this tip gave me picture again. My only issue now is that now I’m stuck at 60hz and I used 100hz. Hopefully Apple will fix this in the next update.
I have the same issue with my external monitors after installing this new Tahoe update. How do I uninstall this update? I don't have time to try and figure out what is happening.
macOS 26.1 changed nothing on my end. My 700€ monitor is still useless..
External Monitor Not Detected After macOS 26 Upgrade
