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I have a 27" iMac and I'm using Samsung SyncMaster SA850 as a second monitor (connected via a displayPort cable with a mini DisplayPort adapter).

Everything worked fine until yesterday - the Samsung screen turned black and now it only works with max resolution 1280 x 1024 (native is 2440x1440). I think I upgraded Lion 10.7.2 to 10.7.3 (I don't remember for sure though).

When I connect the monitor to an iMac with 10.7.2 everything works fine. And another iMac with 10.7.3 has the same problem as me.

Any ideas what could be wrong, please?

Thanks! Tomas

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It sounds like you could have inadvertently set (or had set) HiDPI mode.

I cann't remember (or find with the time I have) how to set this, but I'll throw this answer up and make it Wiki so anyone can edit it if they do remember :)

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Well, I tried disabling HiDPI from Quartz Debug Developer Tool, but it did not change anything (screen's still black). – Tomas Brambora Apr 20 '12 at 16:14

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