Second external monitor not detected after macOS Tahoe upgrade on MacBook Pro

I never like to suggest that users delete files as a way of obtaining relief for problems, but some users have seen success from this when nothing else seemed to work for them.

For MacOS 12 Monetary and some later versions, some users got relief by deleting some preferences files, then Restarting:

Library/Preferences/com.apple.windowserver

-AND- whichever they found of theses two files:

~/Library/Preferences/com.apple.windowserver.<UUID>.plist -OR-

~/Library/Preferences/ByHost/com.apple.windowserver.<UUID>.plist

-- where [~] tilde is an active abbreviation for "current user"

-- and <UUID> is a string of hexadecimal numbers unique to your system, grouped as 8-4-4-4-8 hexadecimal digits

Restart REQUIRED after these deletions.

Have you restarted your Mac manually after the upgrade? That seems to fix a lot of issues.

MacBook Pro 2021 16-in display

What processor type? {M-series (plain), PRO, MAX, ULTRA}

what make&model displays?

, and what cables, adapters and docks are being used to make the connection?

Grant Bennet-Alder wrote:

MacBook Pro 2021 16-in display

What processor type? {M-series (plain), PRO, MAX, ULTRA}

If it is a MacBook Pro (16-inch, 2021), the processor must be either a M1 Pro or a M1 Max.

The OP said that they verified that their machine supports three external monitors, which would narrow it down to a M1 Max. (The M1 Pro can handle up to two external displays; the M1 Max, up to four.)

MacBook Pro (16-inch, 2021) - Technical Specifications - Apple Support

• Be certain any Thunderbolt cables, including Thunderbolt-5 cables, are 0.5 meters or shorter, unless you are using a (much more expensive, starting at US$125) ACTIVE Thunderbolt cable.

• Be sure any USB-C cables or adapter/cables are limited to ONE meter or shorter

• Be sure any HDMI cables for up to 4K at 30 Hz are certified PREMIUM high speed cables.

• Be users any HDMI cables for higher resolutions are certified ULTRA high speed cables and your Mac was released in 2023 or later.

if using a Dock, only ONE fully hardware accelerated display is natively supported on a single USB-C Cable out of the Mac. Use of a ThunderBolt Dock is an extreme special case, and must be discussed on a case-by-case basis.

Same problem. I was told - after lots of testing and sending reports to their engineers - that the fault is that of the monitor manufacturer (of course), and I should instruct them to make their monitor compatible with Apple's new operating system.

Total crap. Same with my M3. Since update to Tahoe only one external Monitor is possible.

2024 MacBook Pro M4 chip. Only one monitor working at a time.