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 1.7.2 to 1.7.3 (I don't remember for sure though).

When I connect the monitor to an iMac with 1.7.2 everything works fine. And another iMac with 1.7.3 has the same problem as me.

Any ideas what could be wrong, please?

Thanks! Tomas