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I'm using a MacBook Pro (Late 2011) connected to two Thunderbolt displays. The displays are chained together, and one Thunderbolt connection is made to the MacBook Pro.

I prefer to have the left monitor function as the primary display (I want the dock to live on the left). Using the Display System Preference Pane, I can set this configuration with no issues.

Every morning, I connect the displays to the MacBook Pro and I disconnect every evening.

Some days, seemingly at random, the display configuration is changed and the right monitor has the dock and menu bar instead of the left.

Is there any way to ensure the display configuration remains the same?

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Just for the record, could you clarify, (1) you have the brand new current (March 2012) MBP, and I guess it is either the 15 or 17 inch? (2) you DAISY CHAIN the two monitors (the MBP has only one Thunderbolt port) ? {To be clear - there is a lot of info on the internet that supposedly you CANNOT daisy chain apple thunderbolt monitors: supposedly, with thunderbolt monitors can only be the "end" of a chain. But perhaps this rumour is complete nonsense?} (3) Finally can I ask you, to be clear all three monitors are logical displays, is that right? i.e. NOTHING is mirrored - you can wipe your mous – user19967 Mar 11 '12 at 13:36
MBP is new/current, but from February, 17-inch. Monitors are daisy-chained. All monitors are logical displays, though I shut the MBP and don't use it's monitor when connected to the external displays. – jro Mar 11 '12 at 22:57
Dear JRO, to be clear, (A) if you DO open the MBP, do you the get three totally separate logical monitors? also as a test ... – Joe Blow Mar 12 '12 at 7:11
... (B) when you unplug, if you unplug the last monitor, THEN the middle monitor, and only THEN sleep or shutdown the MBP .. in fact does that result in it "not forgetting"? (You should not have to do all that of course, but I'm just trying to figure it out.) I have these same problems erratically with multimonitoriMac situations. Horrible. i can't believe Apple messed this up! – Joe Blow Mar 12 '12 at 7:12
> if you DO open the MBP, do you the get three totally separate logical monitors? Yes. – jro Mar 12 '12 at 17:40
up vote 1 down vote accepted

OSX has to guess which monitor is which, since the EDID info is the same for both (save the serial number, presumably, but OSX doesn't appear to pay attention to that). It usually tries to set things up to the previous setup by looking at how the devices are attached in the connection tree, however with Thunderbolt the connection tree isn't as simple as it was with displayport and USB.

You should be able resolve this by attaching one monitor at a time (ie, unplug the thunderbolt connection for the third monitor, attachthe second, then plug the third back in). However that's less than ideal, especially since all your apps will go back to the main monitor, rather than how you had them set up previously.

When Apple releases a refresh of the thunderbolt display, attaching one of the old ones and one of the new ones should also resolve the problem - they won't appear to be the "same" device, and so OSX will have an easier time keeping track of which is which.

Until that happens, another thing you can try is attaching the third one with a thunderbolt extension cable. It's an active device, and may alter the connection tree enough that OSX will have an easier time keeping track of it.

If that doesn't help, your best bet will be to wait until they refresh the thunderbolt display and upgrade only one of them when they do.

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I had to beat my head against OSX for several months writing a video driver. OSX is smart in most ways, and dumb in some others. The major problem being that when it's dumb there's often no easy workaround. – Adam Davis Mar 20 '12 at 17:40

I suffer the same problem, each morning when I come to the office about 80% of the time my Mac wrongly selects the left most TB display as primary >:@

Getting tired of going to the settings display every time, I wrote a very simple console App to swap the screens for me: "ScreenPhant" (like elephants and memory; dig :-P)

It is really too simple, but I have no time now to make it any fancier (like staying resident and monitoring display changes automatically etc.). If you just start it, it will list the displays and serials. Pass the serial of the display you want as primary and it will set it. If you rename the App like "ScreenPhant_151d050f" it will use the _[serial] for the primary display so you don't need an extra script to launch it.

You can download here: Source is available here:

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Perhaps Display Maid could solve the problem. However, I have not tried it so I'm not sure if it will resolve your issue. There is a free trial available.

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