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I use my MacBook with an external monitor at work and home. Unfortunately, they are both the same model and this seems to confuse OS X.

At work, I have the monitor on the left and at home the right. But each time I move from one to the other, I have to manually change the arrangement.

Does OS X not remember arrangement for multiple monitors, or is using the same model the source of confusion here?

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possible duplicate of What Window Management Options exist for OS X? – user588 Apr 28 '12 at 14:24

OS X tries to remember arrangements, but it relies on monitors identifying themselves appropriately. Cheap brands may not bother to put distinct serial numbers in the EDID information they report, even though this is required by the EDID standard, so OS X will end up recognizing them all as the same monitor.

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Is there any way to get the EDID manually? I would like to verify that is the problem. – Adam Lassek Apr 28 '12 at 1:09
If it's not in /var/log/system from when Core Display starts, you may be able to get it from a program like DisplayConfigX or SwitchResX. – geekosaur Apr 28 '12 at 1:18
I should mention that I have not had a problem with OS X forgetting display layouts, but all the external monitors I have available are different manufacturers; I have no way to verify that OS X is actually using the serial number or perhaps using a hash computed on the EDID or something. (The latter would still amount to the same thing, though, since changing the serial number should change the hash; it would still imply the monitor manufacturer is doing something strange.) – geekosaur Apr 28 '12 at 1:21

I have not verified the EDID, but I have a related problem that might shed some light...

I use the old Apple Cinema 30" HD display at both work and home. Unfortunately due to the desk configuration at the office, I'm forced to set-up my 30" monitor to the left of my 15" MBP w/ Retina, but have he opposite configuration at home (with the 30" to the right of the MBP).

I can attest that my MBP does not distinguish between the two Cinema displays and expect the desktop to span both ways for either of the configuration.

If OS X was indeed using EDID and distinguish the monitors by their serial numbers, I expect I would not see this behavior.

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