Many Apple Silicon processors only support two displays at a time.
For the M4 mini, the limit is either two or three, depending on resolutions. Two with one at up to 5K and one at up to 8K/60Hz or 4K/240Hz). Or three, with two at up to 6K and one at up to 5K, all at 60Hz.
The M4 Pro is similar, but the 3-display configuration allows all three to be driven to 6K resolution (still just at 60 Hz).
But this is new. If you look at the previous generation (M2-based) 2023 Mini, the M2 only supports two displays. You would need an M2 Pro to get support for three displays.
Likewise for the (M1-based) 2020 Mini, which only supported two displays. (Apple didn’t release an M1 Pro mini).
I should’ve looked more closer at your post. I see you have an M4 mini.
if all three of your displays are running at 60 Hz and all are within the resolution limits (6K or less for the first two TB displays, 5K or less for a third TB display, 4K or less for an HDMI display) then it should be working.
If your setup fits these criteria, then I don’t know. But maybe check your displays’ refresh rates, if you haven’t already done so. If one supports refresh rates higher than 60 Hz and is being driven at such a frequency, that would be preventing 3-screen operation.
Thanks so much, not least for taking the trouble to start a thread! I didn’t want to take up anyone’s attention, but am happy to fill in a bit more on this, in case anyone else is experiencing this very strange issue.
This appears to be a software issue rather than (as I initially suspected) a hardware failure on a new Mac. My three-display setup had been working fine until I booted up yesterday morning, having changed nothing that you’d expect to impact this (though there was a Rectangle Pro update I installed the previous day). But the key discovery in a very long day of support calls was that if I created a new Admin user and booted into that, all three displays showed up. That indicates that some seemingly unrelated event or action has tipped something in my configuration into this very strange state where only the first two displays scanned (which are evidently the ones connected over USB, then the HDMI third) are recognised. On the plus side, that means the issue is trackable-down, given time, patience, and systematic experiment. On the minus side, 2001 files had changed since the previous evening’s backup (thanks for that nugget, Time Machine) and it’s unclear where the offenders might be found.
More weird things: the speakers on the third display remain available as a sound output. It’s just the screen that only exists if one of the others doesn’t. It’s also recognised over the separate custom (non-video) USB connection that this particular very idiosyncratic display (Boox Mira Pro e-ink display) uses to access settings over a different cable. (I know…) But it’s not the Mira, because I’ve also tried it with other displays over HDMI.
My initial hypothesis is that there’s something in the video port handling on the M4 mini that is easily broken by some subtle and seemingly unrelated change elsewhere. This particular model is a pretty radical innovation in the range, which is putting a different configuration of ports in places they’ve never been, and I think I’ve broken something that other multi-monitor setups will also find themselves breaking in time. But I could be completely wrong. I haven’t found any other reports of this very unusual issue. They may or may not be about to emerge.
This doesn’t surprise me. Multi-function devices are distinctly separate devices. In your case, video on HDMI, and audio on USB. As far as the computer is concerned, they are two completely separate devices.
I see the same thing with my devices. The display I use on my work computer is one of those “designed for Microsoft Teams” displays. It is a 1080p display, a microphone, a camera and speakers. It connects to the computer via HDMI for the video and USB for the other three devices. If one fails, the others keep working fine - which is good because I normally don’t connect the USB cable, since I have other speaker, camera, microphone hardware that I prefer to use instead.
My guess is still display configuration - especially since a new user account doesn’t show the problem.
Double- and triple-check the display settings for the two working displays. If one is (accidentally) configured for a refresh frequency greater than 60 Hz, then it would kick the Mac over into its “2-display-max” operating mode. The newly-created user may be using a factory-default setting of only 60 Hz on each display.
If that doesn’t work, it might still be a problem with settings, but maybe a normally-not-accessible got corrupted, or there’s an obsolete setting that got migrated forward from the distant past and is now causing problems.
To check this, track down the file containing your display settings. I would expect it to be a plist file stored somewhere under ~/Library. Make a backup copy of the file and then delete it. Then reboot. This should reset the display settings back to factory defaults. If there was a corrupt parameter, that should reset it - then you can use the system settings app to reset your preferences. If it ends up creating a new problem, you should be able to recovery by logging in to your other admin account, restore the backed-up file and reboot again.
Thanks! Yes, that’s pretty much where I’ve got to; I’ve ruled out everything I can in the display settings both regular and accessible through third-party tools, and have been trying to pinpoint suspect files in the Library. So far I’ve just focussed on files with recent modifications, but I’ll probably need to brute-force it by copying whole folders to the new user’s Library and seeing if I can trigger the 2-displays glitch that way. This Mac is at the end of a long chain of migrations and it’s entirely likely there’s some old junk from a past life causing issues in this one; at any rate that’s the current thing to check for.