External Monitor Not Detected After macOS 26 Upgrade

to connect via USB-C, your USB-C cable must be Certified and show the USB-SuperSpeed PLUS logo or the USB-20 logo, and be ONE meter or shorter. Longer cables could cause transmission errors, which would cause the display to drop out.

Over DisplayPort or USB-C, provided the cables are adequate spec and not too long, standard 4K 8 bits/color displays support up to 75 Hz without compression, and likely higher with Display Stream Compression.

<< Stop sending people on a wild goose chase for changing their USB cables. It worked before the upgrade, but stopped working after the upgrade - it has absolutely nothing to do with the cable. >>

The Apple standard for its built-in hardware-accelerated displays, makes them suitable for full-motion video for production/display of cinema-quality video with NO dropped frames, and NO dropouts or partial-blank scan lines due to memory under-runs or other issues. 

More recent versions of MacOS has ben ever more demanding about transmit errors, which can be caused by cable that are not quite high enough specs.

If your cables are already up to that spec, then all you will need to do is is INSPECT your cables for those markings, and when found, THEN you can eliminate that as a possible issue.

Some readers have reported that bringing the cables up to the required spec solved their problem, so I will continue to post that advice.

I had this same issue with a Mac Mini M4 after upgrading to 26.1 from 26.0.1. I have 3 monitors and the 3rd one (at 3440x1440 resolution) went blank. That monitor was connected via a HDMI cable that had a HDMI to USB-C adapter on it. Switched that out to a Thunderbolt cable and that didn't fully resolve the issue. There was a slight improvement: the 3rd monitor went from not being recognized at all by the Mini to the monitor being recognized but no signal reaching the monitor. Finally I found this page: How many displays can be connected to Mac mini - Apple Support. and discovered this tip: "Connect the display with the highest resolution first.". So I moved this blank monitor to the first USB port in the back with the USB-C cable (didn't work) and finally the HDMI with HDMI/USB-C adapter, and it worked. Not sure why this was an issue, but I'm happy it is resolved. Good luck!

"External Monitor Not Detected After macOS 26 Upgrade: 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."

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Verify the Connection:

Just curious: How are you verifying this connection? To, see if the monitor is even detected,...

1. Hold Down: the Option key
2. Click: the Apple menu
3. Choose: System Information or System Report
4. Select: Graphics/Displays and USB from the list on the left
5. Look For: the monitor

that Samsung CH890 display is a 3440 by 1440 display, like a 4K display with the bottom half missing. it supports 8 bits/color and refresh rates up to 100 Hz.

interfaces include:

2x HDMI 2.0

1x displayport 1.2

1x USB-C providing up to 65W power

to connect via USB-C, your USB-C cable must be certified and show the USB-SuperSpeed PLUS logo or the USB-20 logo, and be ONE meter or shorter. longer cables could cause transmission errors, which would cause the display to drop out.

over DisplayPort or USB-C, provided the cables are adequate spec and not too long, standard 4K 8 bits/color displays support up to 75 Hz without compression, and likely higher with Display Stream Compression.

I'm having some massive problems after updates too. I have a wavlink wl-ug63pd25 dock that I was using with a quad monitor setup (with 1 monitor plugged directly into the m4 macbook's hdmi port) and this worked for all day every day work usage for months. As soon as I upgraded to 15.7 the dock stopped being able to connect. I tried going up to Tahoe to see if that fixed it, same behavior. If I turn off the setting that automatically allows USB connections when the Mac Book is unlocked, then I get prompted every 15 seconds or so to allow the connection, and then the prompt disappears before I have a chance to click the button to allow. I've tried generating logs with this command:

log stream --style syslog --predicate '(subsystem == "com.apple.usb") || (eventMessage CONTAINS[c] "USB") || (eventMessage CONTAINS[c] "XHC")'

It's a bit cryptic, but it basically just shows the device constantly disconnecting and reconnecting. I have a second dock of the same model, and tried swapping cables and docks and did really everything you can think of to troubleshoot this type of thing at least 10 times. I tried playing with the displaylink driver versions too, but I don't think the dock is even staying connected long enough for displaylink to initialize. I also tried uninstalling displaylink altogether and just trying to use the usb and rj-45 ports on the dock, which should work without any driver/software installation, that doesn't work either.

One thing that does halfway work, is using a C-to-A cable and then converting it back from A-to-C with an adapter at the laptop port side. But this results in severely reduced throughput and I can't use all 3 monitors anymore.

I think it's extremely obvious at this point that something changed with the USB port management software in these recent updates. A lot of people are reporting issues, and for everyone reporting issues there's probably at least 10 others impacted who don't have time to tshoot this kind of stuff. It would be great if we could at least get some acknowledgment from Apple on this and explain what might have changed, and if we can expect some kind of fix in 26.1 or what the deal is.

I'm going to go ahead and order a new cable just to test with, even though I doubt that the wavlink one provided with the dock is any worse than what I can find from a third party. I've lost a lot of time and productivity due to this issue and I'm regretting my choice to "think different" and give apple a chance. I've been using quad monitor setups on Windows for decades and have NEVER had issues like this. It's ridiculous that this is what you get when you drop $1700 on a so-called "high end" device.

<< wavlink wl-ug63pd25 dock >>

The Apple standard for its built-in hardware-accelerated displays, makes them suitable for full-motion video for production/display of cinema-quality video with NO dropped frames, and NO dropouts or partial-blank scan lines due to memory under-runs or other issues. This requires a hardware rasterizer/display-generator for each fully-accelerated display, supported by Huge memory bandwidth to refresh each display 60 or more times a second. 

Because of the bandwidth required for modern displays, Apple DOES NOT support multiple Hardware-Accelerated displays on ONE USB-C port. On a genuine Thunderbolt port connected to a ThunderBolt Dock, you can support as many as two, provided bandwidth is available for both.

The dock you describe supports ONE fully hardware-accelerated display. Additional display are supported using DisplayLink.

When you upgrade MacOS, you may also need to upgrade the DisplayLink Drivers. These most recent version available on the Wavlink web site is version 13, released 28 July 2025. that may be the lasted one compatible with the Wavelink Dock. But that does not appear to be the current version, 14, provided by Synaptics, the developer of DisplayLink.

You should contact Wavlink for guidance.

https://www.synaptics.com/products/displaylink-graphics/downloads/macos

(continued)

Change thunderbolt mode not working for me, but what works:

• Connect monitor through HDMI and set refresh rate to 60Hz
• Connect through Thunderbolt -> works flawlessly on 3440x1440, 120Hz, 30bit

May be some wrong setting for Thunderbolt was stored on Mac 🤷

🛑 RESOLVED: M3 Pro (macOS Tahoe) HDMI Port Not Detecting External Monitor (ViewSonic VX3276-MHD-3)

Setup: MacBook Pro 14" M3 Pro (Built-in HDMI port) → ViewSonic VX3276-MHD-3 Monitor

macOS Version: macOS 26.1 (Tahoe)

Problem: After upgrading to macOS Tahoe, the built-in HDMI port failed to detect the external monitor. The monitor showed a black screen despite working perfectly fine on the previous OS version (Sequoia). Standard troubleshooting failed to resolve the issue.

Part 1: What DID NOT Work (Do NOT Waste Your Time)

The core issue is a highly sensitive driver protocol in the M3 Pro's HDMI 2.1 port under macOS Tahoe, which rejects the handshake with older/standard HDMI 1.4 monitors when corrupted display preference files are present.

Here are the fixes that failed:

1. Standard Reboots/Re-plugging: (Obvious, but still failed).
2. OSD Settings: Disabling Adaptive Sync, FreeSync, or HDR on the ViewSonic monitor.
3. Basic Software Workarounds: Holding Option (⌥) while clicking "Detect Displays" in System Settings.
4. Display Management Software (BetterDisplay): Even forcing redetection, reinitialization, and creating a Virtual Screen failed because the system was likely blocking the necessary permissions or corrupted files prevented a clean driver launch.
5. Initial Cable Upgrade: The original HDMI cable, even at 1.5m, was insufficient due to lack of certification/quality.

Part 2: The Final, Successful Solution

The solution required a combination of the highest-quality hardware and a deep-level software reset to purge the display configuration cache.

1. Hardware(Cable) Upgrade (Essential Prerequisite)

The M3 Pro's new port requires an absolutely clean signal. The old cable was not good enough.

• Action: Replaced the original HDMI cable with a Certified Ultra High Speed HDMI 2.1 Cable.
• Successful Cable Used: Zebronics HAA2021 (HDMI 2.1, 48 Gbps, 2M).
• Why it worked: This cable provides the high signal integrity required by the M3 Pro's HDMI 2.1 port, eliminating hardware failure as a possibility.

2. Deep System Reset (The Critical Step)

Even with the new cable, the Mac still failed to connect until the corrupted display files (.plist files) were forcibly removed from the operating system.

🚨 WARNING: Proceed with caution. These commands remove system display settings and require a restart. Do not attempt this unless you have exhausted all other options.

Disconnect the Monitor: Ensure the new HDMI cable is unplugged from the MacBook Pro.

Open Terminal: Go to Finder → Applications → Utilities → Terminal.

Run the Commands: Execute the following three commands sequentially, pressing Enter after each one. The system will prompt you for your admin password for the sudo commands.

rm -f ~/Library/Preferences/ByHost/com.apple.windowserver.displays*.plist

sudo rm -f /Library/Preferences/com.apple.windowserver*.plist

sudo killall Dock

Restart the Mac: Immediately go to the Apple menu () and select Restart.

Reconnect: Once the Mac has fully rebooted and you are logged back in, plug the new HDMI cable back into the M3 Pro's built-in HDMI port.



Final Result:

The external monitor was instantly detected and showed the picture correctly for the first time since the macOS Tahoe upgrade. The Display Settings pane correctly showed both the MacBook and ViewSonic monitors.

I hope this detailed guide saves other M3 Pro users the extensive time and frustration I experienced.

hlsu wrote:

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.

Nice to see things are working smoothly...

I would shut down and restart more than once to sort glitches on an upgraded macOS.

ref: Connect one or more external displays with your Mac - Apple Support

With all due respect this has absolutely nothing to do with USB-C cables, but it is a buggy release from Apple. According to the Mac User Guide there are absolutely no changes in the requirements for thunderbolt / usb-c cables upgrading from MacOS 15 to MacOS 26.

Use USB-C cables with Mac - Apple Support

Stop sending people on a wild goose chase for changing their USB cables. It worked before the upgrade, but stopped working after the upgrade - it has absolutely nothing to do with the cable.

DisplayLink technology creates a "fake" display buffer in RAM, sends the data out over a slower interface to a stunt box with DisplayLink custom chips that put that data back onto a "legacy" interface. It is not a true "accelerated" display, and it can suffer from lagging. Just adding the DisplayLink Driver is not adequate to get a picture -- you need a DisplayLink "stunt-box" or a Dock that includes DisplayLink chips.

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It may be acceptable for a second display showing slow-to-change data such as computer program listings, stock quotes, or spreadsheets, but NOT for full motion Video, not for Video editing, and absolutely not for gaming. Mouse-tracking on that display can lag, and can make you feel queasy.

In a pinch, it may even play Internet videos (as one user put it) “without too many dropped frames".

If you are only doing program listings spreadsheets, stock quotes, and other slow to change data, DisplayLink can work for you, but requires you to make some strong compromises.

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It is really nice to know that you can use a DisplayLink display if you MUST have an additional display for some of the types of data I mentioned. But that is NOT the same as the computer supporting a second, built-in, Hardware-accelerated display.

These displays depend on DisplayLink software, and are at the whim of Apple when they make MacOS changes. There have been cases where MacOS changes completely disabled DisplayLink software, and it took some time for them to recover.

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I think the Big Surprise for a lot of Hub/Dock buyers is that they thought they were getting a "real" display, but actually got a DisplayLink "fake" Display. If you got what you expected in every case, I would not use such pejorative terms to describe DisplayLink.

I no longer have the ability in settings display to expand to an external monitor in settings/display however the screen sharing icon on the upper taskbar works to send to an external monitor. You may need to enable screen sharing to get the icon.

Works on both my M1 and the M4 macs.

For my M2 Macbook Air running Tahoe, abhiraj_chatterjee's solution was mostly correct. In Tahoe, the system preference file has a simpler name:

/Library/Preferences/com.apple.windowserver.displays.plist

The user preference file name still includes the long <UUID>. This article in the plugable knowledge base explains the problem very well.

Note: educated myself (was too late to edit my post) and looks like flipping the cord forced a fresh mapping of sorts (like what switching from sleep to wake, etc was supposed to accomplish)