Yosemite does not recognize external displays...

Hi SS7886247,

I see that you are having issues with your external display since the update to Yosemite. I would like to suggest that you perform a PRAM reset to see if this troubleshooting step affects the issue:

OS X Yosemite: Reset your computer’s PRAM

http://support.apple.com/kb/PH18761

There are some further troubleshooting steps found in this article as well:

Apple computers: Troubleshooting issues with video on internal or external displays - Apple Support

http://support.apple.com/us-en/HT201177

Thanks for coming to the Apple Support Communities!

Regards,

Braden

Hi I did the same thing, as well as fixing the permissions Permissions problem in displaypolicyd

and also downloading the thunderbolt 1.2 firmware update from app store.

PRAM did actually help, but I had to unplug the dvi cable from the adapter, then plug it in again after doing the PRAM reset.

When I put cable back in, it worked.

Hi again SS7886247,

Thank you for using the Apple Support Community. Due to the nature of your issue or question you may find more information by using another one of Apple's support resources - https://getsupport.apple.com/GetproductgroupList.action.

Cheers,

Braden

Hi again,

you don't install PRAM, you have to reset it, the instructions are here OS X Mavericks: Reset your computer’s PRAM

as for the displaypolicyd, you need to type this into the terminal:

sudo diskutil repairPermissions /

and for the update, there is no such app, but the update is in the tab updates in AppStore, under Software Updates, just click 'More'

Has there been any resolution to these issues? I just upgraded a late '12 Mac Mini to Yosemite and am now also having similar difficulties, utilizing TWO Video Output options simultaneously. With 10.8.5 I was using my Thunderbolt port to adapt to VGA which goes to a Stardom KVM and then to a professional 1920x1080 monitor, while at the same time utilizing the HDMI port to connect to an AJA HA5 convertor (which adapted that video signal to SDI).. Everything worked seamlessly before this upgrade. Using exactly the same hardware my current problem is that my Mini(s) now fails to correctly recognize my monitor that is down the line from the Thunderbolt to VGA adaptor. That monitor now only gets recognized at some pathetically useless lower resolution, while the HDMI video Out seems to work just fine. In fact, the HDMI out is what the Mini now prefers. WHY??? I likely am missing something, I know.... The Displays preferences is sheer misery to navigate between the two and I have thus far failed to get the two monitors (well, one is actually an SDI router with multiple monitors down the line... and that is from the HDMI Out, which gets converted) to work the same way twice after a re-start. I spent a few hours feeling helpless today. Tomorrow I will do the VRAM reset (forgot to do that today), but I am not so hopeful. Anyone?

This worked for me when my MacBook Air Mid 2013 El Capitan wouldn't connect to my Epson 4650 projector via VGA adapted to Mini DisplayPort.

Unplug from the projector, go to "System Preferences>Displays>Color"...Find your Projector (might work with monitors also) in the"Display profile" list and delete it.

If built in display color goes wonkie select the top profile, mine was " Color LCD"- or create a new one via the "Calibrate..." button.

Reconnected to my projector and it was recognized.