El Capitan does not detect my external display

Same situation here, tried everything still no joy. Macbook air mid 2012 running el capitan 10.11.3 thunderbolt to HDMI doesn't recognize TV that used to work fine before upgrade to El Capitan.

I too have this problem only started when i upgraded to EL Capitan and ive tried the holding down the ALT key detect displays dosent work with my TV or my HDMI monitor, this is on a Mid 2010 27" IMac.

Need a resolution Apple???

Just to add my cable had NEVER been disconnected until it stopped working when i did the EL Capitan upgrade, so its not a bad cable!

I had this problem too, but it turned out my monitor cable was plugged in upside down! 😕

You would think with would be impossible with an apple product, but this has happened more than once for me and I've seen others do it too.

It hadn't been unplugged?

I got to thank you because all this time I had issues with El Capitan and as soon as I read your post I thought you might be right I have not check the cables,so I started with the HDMI bought another one and sure enough the Imac found second monitor!

Thank you again!

I've had this issue with multiple Dell monitors UP2715K 4K & P2715Q 5K, in Yosemite and El Capitan on a Macbook Pro Retina 2014. Put the Macbook to sleep manually, close the laptop, or lock the screen manually, and then attempt to wake the machine there is no output to the monitor. I've also used a series of different DP to Thunderbolt cables.

The only workarounds I've found are to remove power from the Monitor for a few seconds (works 10% of the time), or restart OSX (works 90% of the time).

Recommend double checking that your Thunderbolt cable is connected the right way. My external Thunderbolt Monitor wasn't being recognized by my iMac2015, then I reversed the cable and it came back.

I solved my problem so far by simply installing this app https://itunes.apple.com/pt/app/display-menu/id549083868?l=en&mt=12 from the Mac AppStore.

The screen flashed and I got image.

Guys i don't know if its gonna work for you, but look a my answer and this discussion, hope it will help Re: my macbook pro does not detect external display anymore

Actually the screen went black again later. So, the app is cool but it didn't fix the problem for me.

What fixed the problem (so far, after a few days) was to lower the resolution of the 2nd monitor.

I run into same problem as you too.

My Macbook air purchased in Y2015 Mar was working fine with Dell 24" U2417. This monitor come with display port. My MAC plug and play well.

Yesterday, I bough another Samsung 27" C27F390, this monitor only has HDMI, so I got to buy a HDMI to Display Port adapter (Brand = Daiyo) . When it connect to Macbook air. It just not able to detect by Mac.

I do all the Mac reset trick and it just won't work.

Thanks for sharing your experience and that confirm I have the same problem as you and highly chance caused by the adapter.

Wonder what brand is the adapter you use.

I have an old adapter for my cinema display, which is Mini DisplayPort to DVI, and I'm having intermittent problems so I was very interested in this list of compliant adapters. When I look at mine I see that it is "designed by Apple in Cupertino" but on the approved list there are no Apple adapters! Can this be?

I am really disappointed. Neither my external monitor, nor my superdrive is recognised after upgrading to El Capitan. It is taking Apple a very long time to work on the problem, since I've seen elsewhere that they are investigating. I took my superdrive to the iStore in Pretoria (South Africa) where it was "tested" by the technicians. I was advised to buy a new one, which I did. The new one did not work either. Many hours later, I took my husband's Macbook Air. He had not yet upgraded to El Capitan. And yes, the superdrive was working.