External Monitor Not Detected After macOS 26 Upgrade

Using another cable "fixes" the issues. Nevertheless it is kind of strange that the problem only appears on macOS 26. I tried a different MacBook with macOs 15 and it worked flawlessly.

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.

Hi! The only thing that worked for me was flipping the cord so each end is connected to the opposite port

Think it forces a re-mapping of sorts

"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)

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.

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)

Make sure the monitor is powered on before connecting it to the Mac. FWIW, I have a 16" M4 Pro MacBook Pro and I'm typing this on an external display, although mine is connected via Thunderbolt (TB cable to a Stone Pro TB3 dock then a TB cable to TB3-capable LG 5K:2K display).

DaveGarratt wrote:

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

More recent version of MacOS have been increasingly demanding that there be NO transition errors. If the display in question has a very slightly faulty USB connection at the display, then it is plausible that the increased demands of MacOS 26 push it from being marginal to failed.

One reason I spend a lot of time here is to learn stuff. I want to know what display has this "known history" of USB port issues. I would like to read about it and see if there might be ways to work around that problem

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I have had my share of USB-C cable issues in the past, and swapping cables may be worth trying, but I don’t think this is the whole answer.

There is something not right with the update. I have 2 x USB-C displays connected directly to the ports on a Mac mini M4 pro. Worked perfectly before Tahoe. Now only one monitor will connect unless I switch the other one off and on again. EITHER monitor works when the other is switched off. When both are on, the second display does not connect. Doesn’t matter which one I power on, it works if the other is switched off. Swapped cables between the two monitors and switched each off separately. The problem does NOT follow the monitor or cable or port… Can’t help feeling this is a related issue.