I have a 21.5" mid-2010 iMac with OS X Yosemite 10.10.3. For years now I've had my Sony-TV connected to the iMac via an HDMI-cable and a HDMI-to-Mini Displayport -adaptor. I've had zero problems with this setup. However, last night I wanted to watch a movie from my iMac via the Sony-TV but the screen was all black. I then went to my iMac and discovered that it had stopped detecting the Sony-TV. I did the following procedures: Switched all devices off and reconnected all the cables Checked that the Sony-TV's HDMI slot was working and that the HDMI-cable was working (connected it to a Windows-PC) Ran the Onyx Automation procedure Did SMC and PRAM resets Booted the iMac from a clone drive (taken when the HDMI-connection between the Sony and the iMac was working) Alas, nothing helped. I have read that sometimes, after OS X updates, people have trouble with their external displays but this happened well after the 10.10.3 update and since the update I had used the Sony-TV with the iMac without a single problem. Do you have any suggestions?