How do I make my Studio remember which monitor is which?

I'm having the same problem on a new Mac Studio with two identical Samsung monitors connected via Thunderbolt to DisplayPort cables. They look lovely and run at 144Hz, but switch sides randomly on every wake from sleep. I tracked down how to extract the EDID info, and it's very simple in a terminal:

    ioreg -l | grep EDID

For me this shows my two monitors:

    | |   |   |   "DisplayAttributes" = {"SupportsSuspend"=No,"MaximumRefreshRate"=144,"SupportsActiveOff"=No,"PortID"=32,"ProductAttributes"={"ManufacturerID"="SAM","YearOfManufacture"=2018,"SerialNumber"=810889805,"ProductName"="C27JG5x","AlphanumericSerialNumber"="HTOKC02346","LegacyManufacturerID"=19501,"ProductID"=3928,"WeekOfManufacture"=51},"MaxVerticalImageSize"=34,"MaxHorizontalImageSize"=60,"HasHDMILegacyEDID"=No,"Chromaticity"={"Red"={"X"=44352,"Y"=20736},"Green"={"X"=18048,"Y"=43328},"Blue"={"X"=9984,"Y"=4032}},"DefaultColorSpaceIsSRGB"=No,"NativeFormatHorizontalPixels"=2560,"DefaultWhitePoint"={"X"=20544,"Y"=21568,"Gamma"=144179},"SupportsVariableRefreshRate"=No,"AspectRatio"=15,"MinimumRefreshRate"=50,"WhitePoints"=({"X"=20544,"Y"=21568,"Gamma"=144179}),"PreciseAspectRatio"=115652,"ContinuousFrequencySupport"="None","SupportsStandby"=Yes,"NativeFormatVerticalPixels"=1440}
    | |   |   |   "EDID UUID" = "4C2D580F-0000-0000-331C-0104A53C2278"
    | |   |   |   "DisplayAttributes" = {"SupportsSuspend"=No,"MaximumRefreshRate"=144,"SupportsActiveOff"=No,"PortID"=48,"ProductAttributes"={"ManufacturerID"="SAM","YearOfManufacture"=2018,"SerialNumber"=810889805,"ProductName"="C27JG5x","AlphanumericSerialNumber"="HTOKC02337","LegacyManufacturerID"=19501,"ProductID"=3928,"WeekOfManufacture"=51},"MaxVerticalImageSize"=34,"MaxHorizontalImageSize"=60,"HasHDMILegacyEDID"=No,"Chromaticity"={"Red"={"X"=44352,"Y"=20736},"Green"={"X"=18048,"Y"=43328},"Blue"={"X"=9984,"Y"=4032}},"DefaultColorSpaceIsSRGB"=No,"NativeFormatHorizontalPixels"=2560,"DefaultWhitePoint"={"X"=20544,"Y"=21568,"Gamma"=144179},"SupportsVariableRefreshRate"=No,"AspectRatio"=15,"MinimumRefreshRate"=50,"WhitePoints"=({"X"=20544,"Y"=21568,"Gamma"=144179}),"PreciseAspectRatio"=115652,"ContinuousFrequencySupport"="None","SupportsStandby"=Yes,"NativeFormatVerticalPixels"=1440}
    | |   |   |   "EDID UUID" = "4C2D580F-0000-0000-331C-0104A53C2278"

and you can see that the two monitors produce absolutely identical output – same serial number, same UUID. Samsung needs a slap, or we need a way to anchor monitor ID to physical port – but the port ID is right there in the output, so macOS could tell them apart anyway! So while Samsung shouldn't use duplicate serials, macOS could work around this. Searching for this 810889805 serial number finds several people who apparently have the same one as I do.

I previously had these same monitors connected to a Mac Pro with an RX580, both connected via HDMI, and while they had other issues, they didn't switch sides.

On Friday I completed the third of three calls with Apple Support Tech Brennan, working out of Canada. The calls totaled about 5 hours, of diagnosis, testing configurations, cables, settings, reboots, firmware, etc. My impression was he did a thorough workup. To make a long story short, this is now a "known issue." He and engineering are aware of this thread and cross-referenced threads as above. It appears most likely that a software alteration will be needed in a future release, since the issue is being categorized, I believe, as a flaw that needs to be addressed by engineering. I am not aware of a timeline and do not have direct access to engineering, but the process of interview and feedback and information relay was conducted with energy and efficiency, so we'll see. Surely Apple will recognize that many Studio customers will be looking for a multiple monitor arrangement. Fingers crossed for an early solution. Aloha to all, T.A. Gill; Honolulu

I have a similar problem. Apple Studio Ultra, top of the line. 12.3.1. One Apple Studio Display in the center, and two Samsung 27" displays with 4K to the left and right of it. The Samsung displays are a few years old but never misbehaved on my Mac Pro. "Every log in is an adventure"... true. I have to go back to display preferences and rearrange the screens so my cursor will track correctly left to right after almost every startup ... and sometimes after a mere sleep as well. Every display is on its own rear Thunderbolt port. I've tried rearranging what ports are used for what screen, but to no avail. My $7000 machine cannot remember where I designate each screen... what's up with that?

I just got off the phone with a senior support person about this issue. I included this thread in the discussion about it.

She read through my thread here ( Mac Studio + LG UltraGear QHD/2k = Worksp… - Apple Community ) with me on the phone and acknowledged this appears to be an issue with my Mac Studio and how it handles the video output.

I narrowed it down to it being an issue with USB-C to DP with my specific setup, on my KVM.

All my other PCs and even Mac Mini (Intel w/eGPU) works fine for all my monitors when the are all using DP to DP cables only.

My Mac Mini will have these monitor issues if I use any USB-C to DP like my Mac Studio, so I know there's something wrong with the method of connecting video, maybe monitor EDID info is not being sent back to the Mac through this connection, and that's why I have to rebuild my work environment each time I wake the Mac up.

Anyway she's calling me back tomorrow evening after an Engineer has a look at this. I was very detailed in my thread so hopefully they'll have all they need.

I'm sorry, but to pass the buck to the display manufacturers is nonsense. Are you trying to say that if I've bought a new mac studio, then I won't be able to use 2 brand new monitors from the same manufacturer because the mac studio might not be able to tell them apart?

if I've got one monitor connected to my hdmi port, which I want to use as my main monitor, and another connected to a usb-c port which I set as my extended monitor then surely ANY mac in 2022 should be able to remember which is which? Or am I expecting too much?

When you post here, you are "preaching to the choir". Readers are all other users like you. Apple movers and shakers do not come here looking for things to do -- they are already busy.

You can leave feedback here:

Product Feedback - Apple

You can open a support "ticket" here:

Get Support

or you can contact the display maker and let them know you will need to send back their inferior displays unless you get a firmware upgrade that fixes this.

You can leave feedback here:

Product Feedback - Apple

You can open a support "ticket" here:

Get Support

or you can contact the display maker and let them know you will need to send back their inferior displays unless you get a firmware upgrade that fixes this.

I am having the same issue with three monitors as I posted here: Mac Studio + LG UltraGear QHD/2k = Worksp… - Apple Community

I don't know what the solution is, but when I had a chat with IOGEAR over the KVMP I am using (for two PCs and a Mac Studio), they indicated that the issue could be the fact that using a USB-C to DP adapter is the problem. Of course this itself is a problem since there is no actual DP outputs on the Mac Studio.

And I think they're on to something as my Mac mini (late-2018 Intel + eGPU / Radeon RX 580) doesn't have a problem with remembering which monitors are which when connecting the eGPU to the KVMP, because it's direct DP to DP and not USB-C to DP.

Hopefully someone more technical on this can look into it and maybe find some kind of solution.

So, that lasted for about a day. Just been for dinner, returned and the screens gone weird again. The second monitor didn't even come on!

So, I'm trying this next. I've de-selected all the options in mission control

I'll try this option and see how long it lasts. It might not be suitable for your use but I don't have a problem with this set up..

Thanks Pixelco. I loved your toothbrush analogy by the way. I think it was you that put that there anyway. lol. Had me laughing for quite a while.

There was an issue like this quite a while ago. The essence of the problem was convoluted.

To get a Mac display to become active, you need the Mac to query the display, and the display to answer with its name and capabilities. One of the other things typically included in this EDID information is the serial number of each display.

But since most users have PC, not Mac, they simply slam the resolution and refresh settings to whatever they like and let it rip. This does not provide the appropriate feedback to the manufacturers to get the EDID information perfect every time.

In the previous case, investigation found that the display was not providing its serial number in the correct field. Because of that, the Mac could not tell the twin devices apart, and they constantly swapped settings.

The solution has to come from the display maker. Or a work around is to get away from twin displays that are indistinguishable from the point of view of the EDID information they provide.

Further to my note above: following the lead suggested by Grant, I located another Samsung 27" 4K display in the office. This other display was purchased in a different year than the two described above. It looks and behaves the same and has the same specs. Using this one in place of one of the ones that was getting confused, is a step forward. In preferences/displays, in the arrangement panel, now the two Samsung displays have different names. This may improve behavior.

This morning's boot up went fine. The displays showed up in the correct orientation. Again, this is with the Studio Display in the middle and two 4K Samsung displays of different vintage to the left and right. Now, the Samsung displays have different names. Now, one is "U28E510" and the other is "U28E590". I think the latter is the newer one. Previously, using two displays of identical vintage , the names showed up as "U28E510(1)" and "U28E510(2)". My conclusion is that it helps to have screens with different names.

Another thing: previously, with the two Samsung displays both named "U23E510", the dock used to hop around from display to display on reboots or awakening from sleep. That has not happened using one "U23E510" and one "U23E590".

I've been fortunate because my office has several displays with identical specs to choose from.

I'll report again if this seemingly-happy resolution is not stable.

I do not speak for Apple. Everyone here is another User like you, unless their Avatar is tagged with "Apple Community Specialist".

I am not advocating you use any particular manufacturer's displays. The issue occurs when you use TWIN displays where the manufacturer has deviated from the VESA standard response format.

Your Mac sends a query to the display, and the display is supposed to respond with its name and capabilities (using the current connection), as well as its serial number in the VESA-designated serial number field in the response.

If the display manufacture does not supply the serial number in the correct field, then your TWIN displays are indistinguishable, and the Mac may accidentally shuffle them.