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
Just to help others Deep System Reset by abhiraj_chatterjee worked here too.
So the culprit was corrupted display preference files left behind by the macOS Tahoe upgrade. Not the hardware, not the cable, not the display switch. macOS had cached bad display settings and was silently rejecting the connection every time based on that stale data.
To recap the full journey for anyone who ever finds this:
Unplug monitor and run The Deep System Reset script posted by abhiraj_chatterjee:
bash
rm -f ~/Library/Preferences/ByHost/com.apple.windowserver.displays*.plist
sudo rm -f /Library/Preferences/com.apple.windowserver*.plist
sudo killall Dock
Then restart, plug back in after restart -> display works again fine.
Same issue here with Macbook Air M3 and Dell U3425WE, after update to 26.1 it is not possible to connect Dell monitor through thunderbolt, HDMI works OK.
If the issue just appears after upgrading to 26.1, it would be same as External monitors can't be detected after… - Apple Community.
Hello, I may be experiencing a similar issue. I have used a TV for my display for ages and after updating to macOS 26 yesterday, the screen is blank. When I plug the same exact cord into a computer monitor, it displays properly. So maybe there is an issue relating to using a TV as a display?
My computer is a Mac Studio M1 Max, my TV is a Vizio 65", cables are HDMI [regular?], and I am going through an Onkyo audio receiver.
As with the OP, this same setup was working fine before upgrading yesterday, and still works fine when plugged into a computer monitor instead of my TV.
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 too complicated.
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
The external monitor was not shown up in the system information (Graphics/Displays).
[Edited by Moderator]
Before the upgrade to macOS 26, the external monitor was connected (via USB-C) as an extended display and it was working fine.
HDMI cables you want for HDMI-only Displays (higher resolutions than 720p TV sets) are marked as Certified with an anti-counterfeiting tag and are labeled:
"PREMIUM High Speed HDMI cable" or that + "with Ethernet" (up to 4K at 30Hz) --OR--
“ULTRA High Speed HDMI cable" or that + "48G" (supports higher resolutions and backward-compatible)
Cables with No Certification tags are good for your standard 720p TV set, and not much more.
HDMI was invented for HD TV sets. it works great at its original resolution of 720i or 720p. At higher resolutions, it quickly develops issues that are complex to solve, and the cables and adapters required to solve are NOT intuitive.
I don't understand the logic behind recommending that the cable is the issue. The cable worked moments before I did the macOS 26 upgrade, with the same computer, monitor, and cable. Once the software was updated, the setup didn't work. This thread has many views compared to all other Tahoe topics, likely meaning others are searching for this same issue.
The USB-C to USB-C cable I was using came with a monitor. I also tried a USB-C cable that comes with other Apple products. Both have the same result.
I will try to order a Thunderbolt cable. If that solves the issue, I will report back. However, it's silly that a software update now requires purchasing a different type of cable.
<< the Mac refused to see it anymore, restarts, unplug and plug back in, different HDMI cables, nothing. >>
Using a different cable is a fix for BROKEN cables. It does nothing to improve a sub-par cable into an acceptably-good working cable.
Cables "found" behind your TV set are not likely to be high enough spec to be usable for a modern display connected to a modern Mac.
HDMI cables you want for HDMI-only Displays (higher resolutions than 720p TV sets) are marked as Certified with an anti-counterfeiting tag and are labeled:
"PREMIUM High Speed HDMI cable" or that + "with Ethernet" (up to 4K at 30Hz) --OR--
“ULTRA High Speed HDMI cable" or that + "48G" (supports higher resolutions and backward-compatible)
Cables with No Certification tags are good for your standard 720p TV set, and not much more.
HDMI was invented for HD TV sets. it works great at its original resolution of 720i or 720p. At higher resolutions, it quickly develops issues that are complex to solve, and the cables and adapters required to solve are NOT intuitive.
I have the same issue. My external Monitor hardly gets detected. It randomly works when I try like 20 Minutes or so. But if I lock my MacBook and the Monitor switches into power saving mode it won´t be detected afterwards. This is pretty annoying and started when I updated to macOS 26 Tahoe. I tried different Thunderbolt cables, turning on and off the monitor and MacBook and tried to "detect displays" in system settings.
What's kind of weird is that if the Monitor is connected and is waiting for a video signal I can use the connected devices like keyboard and mouse. So the only problem is the video signal.
<< 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.
External Monitor Not Detected After macOS 26 Upgrade
