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sabarirajan19
User level: Level 1 (14 points)

External HDMI monitor not detected properly after updating MacBook Air M2 to macOS 27.0

I have a MacBook Air M2 (16 GB RAM) connected to a Samsung 27-inch monitor via HDMI. This setup was working normally for a long time.

Yesterday I updated my Mac to macOS Golden Gate 27.0 (26A428). Since then, I have been having several display-related problems.


External monitor issue:

    • Samsung monitor works normally when connected to another USB-C device, so the monitor itself appears to be working.
    • When I connect the HDMI cable to my MacBook, the Samsung automatically switches from Analog to HDMI, so it detects the HDMI connection.
    • However, the Samsung is not properly detected under System Settings → Displays.
    • In System Information → Graphics/Displays, initially only the built-in MacBook display was shown.
    • At one point the Samsung displayed the macOS wallpaper, but the mouse cursor could not move onto the Samsung display and the external display was not functioning as a proper extended display.
    • I also tried another user profile on the Mac, but the problem occurs there as well.
    • I tried restarting the Mac and booting through the startup options/Safe Mode troubleshooting.


Other problems since the update:

    • Switching between user profiles has also become unreliable.
    • Sometimes the login screen gets stuck and I have to force-restart the Mac.
    • Overall, macOS seems to be behaving unusually/buggy since the update.


Mac: MacBook Air M2, 16 GB

macOS: Golden Gate 27.0, build 26A428

Monitor: Samsung 27-inch

Connection: USB-C → HDMI → monitor


Has anyone else experienced external monitor detection problems and/or login/user-switching issues after upgrading to macOS 27.0?


I would especially like to know whether this is a known macOS 27.0 issue and whether there is a recommended fix before I consider downgrading back to macOS 26.


Posted on Sep 16, 2026 6:47 AM

3 replies

sabarirajan19 Author
User level: Level 1 (14 points)

Sep 16, 2026 9:43 AM in response to neuroanatomist

Thank you! I tried the cable/adapter suggestion. I changed both the HDMI cable and the USB-C-to-HDMI adapter, but the issue remained on the Mac.


I also tested both cables/adapters with a Windows PC connected to the same Samsung S24C31x monitor, and both worked normally. So the monitor, HDMI input, cables, and adapters appear to be working.


I then followed your suggestion regarding the WindowServer preferences. I backed up and moved the following file:


~/Library/Preferences/ByHost/com.apple.windowserver.displays.764CE3C6-452C-5C11-A71E-A96A2341ADFA.plist


After restarting the Mac, the Samsung monitor was detected correctly and is now working as an extended display. The Samsung S24C31x now appears in System Settings → Displays, and the cursor can move between both displays.

So in my case, resetting the WindowServer display preference appears to have resolved the external-monitor issue.


Interestingly, I was also experiencing occasional problems with user switching/login getting stuck after updating to macOS 27.0, so I'm going to keep monitoring that as well.


Thanks again for the suggestion, it looks like the WindowServer display preference was the issue in my case.

neuroanatomist
User level: Level 9 (72,871 points)

Sep 16, 2026 7:35 AM in response to sabarirajan19

macOS has become progressively less tolerant of video data packet loss due to sub-par cables. There were several reports of setups that worked in Sequoia not working in Tahoe and likewise in Golden Gate, with nothing changed but macOS. 

 

One recommendation is to try a new cable. Be sure to use high quality cables – HDMI should be labeled Ultra or Premium, USB-C cables should be rated for high-speed data transfer, etc. Also, cables should be as short as possible (≤1 m for most cables, <0.5 m for Thunderbolt 5 cables). That random cable you found in a drawer might or might not work. 

 

Some users have found that the issue was caused by a corrupted preference file. To test that, you can delete the relevant files with the following steps (macOS will automatically recreate a new, clean one):


  • Select Finder > Go menu > Go to Folder, paste the following
/Library/Preferences
  • Delete the file named "com.apple.windowserver"
  • Again select Finder > Go menu > Go to Folder, paste the following
~/Library/Preferences
  • Delete whichever of the two files you find:
    • In the Preferences folder that opened – "com.apple.windowserver.<UUID>.plist"
    • In the ByHost subfolder – "com.apple.windowserver.<UUID>.plist"
    • (Note that <UUID> is a string of hexadecimal numbers that is specific to your Mac)
  • Restart your Mac

 

Source: External Monitor Not Detected After macOS… - Apple Community


neuroanatomist
User level: Level 9 (72,871 points)

Sep 16, 2026 10:30 AM in response to sabarirajan19

Glad that sorted it out for you!

Originally posted at discussions.apple.com/thread/256358182