Yosemite does not recognize external displays...

My Macbook Pro late 2011 was updated to Yosemite and the Moshi adapter HDMI in the Thunderbolt stopped working.

Thanks for posting this. This worked for me. I had to unplug the cable from my computer, turn off the monitor, unplug the power cord for the monitor and then do everything in reverse (plugged in the power cord, turned on monitor, plugged in the cable) and it worked!

Brenda, thank you! I tried a lot of things, including restart, cables etc. I followed exactly the sequence you posted, but still not working. I read in other forum that after the Yosemite some Moshi HDMI adaptors stopped working. I think this can be the case. I bought another HDMI adaptor to test. If it works I will post here.

Power cycling the monitor off and on a few times worked for me when using a HDMI to DVI cable (not just an adaptor, but the full 6' cable that had a HDMI on one end an DVI on the other). This was a non apple cable that was purchased on Amazon: It was a "BlueRigger High Speed HDMI to DVI adaptor Cable" that costs all of about $7. It didn't work at first, but the power cycle did the trick...for now anyway, fingers crossed it stays that way. This is on a 2015 MBP 13".

With that said, I can't get my other monitor to work with an Apple Store purchased Mini Display Port to DVI connector.

Wanted to update this post since my resolution came after I realized that through utter coincidence, the DVI port on my Samsung 213T (yes, it was 10 years old) got fried. It probably wasn't the upgrade to Yosemite that caused it. I had been having periodic power surges coming from one of the dozen or so devices plugged into the circuit causing it to trip at random intervals. Turns out it was the failing DVI port on that monitor. Now I have a shiny new Dell U2415 that works just fine.

I've also been battling with this issue for a couple of weeks now. It still didn't work after upgrading to 10.10.3. At some random point I was able to use the monitor properly (no idea why) and then it stopped again. Today I tried everything again, resetting, unplugging, etc... nothing worked. Then I plugged in a PC to the monitor so I could use its menu and reset to factory settings. Then plugged in the mac using the same cable (http://www.macally.com/EN/?page_id=2552) and it worked! I hope this helps someone and I hope it stays like that for a while... I'll update if things change.

I am also experiencing the same fault. ive done the resets. Thought my display port was faulty. Purchased a new one. Thought my hdmi cable was faulty and so changed it up until I landed on the forum. I'm using a display port to hdmi on my Sony TV. It's freaking annoying.

I have been using a Monoprice cable, MDP to HDMI on my MBP 2011 vintage driving an ACER monitor. I've had no problems with it. Previously I had another older Acer monitor connected to an older iMAC, probably 2006 vintage, using the DVI port (no HDMI) and it worked fine (using a MiniDvi to DVI cable). That computer lost it's battery and I haven't had time to tear it apart to replace the battery. (Why do they hide a battery with a known lifetime inside a computer).

Today I bought a new iMAC 27" Retina with 1 TB Fusion Drive. I want to move the HDMI monitor to the new iMAC and use the DVI monitor with the MBP.

While the new iMAC is downloading the data from my Time Machine MBP backup (takes 2.5 hours) I tried to connect the older ACER monitor to my MBP so that I can use the HDMI ACER with my new iMAC. I could not get this monitor to work with the DVI input. I tried two different cables and adapters. I finally went to a new Monoprice MDP to VGA cable and it works. I'm running the latest Yosemite 10.10.3. My HDMI display worked fine through all the upgrades through 10.10.3. Not sure why there is a problem with DVI. VGA seems to work and is driving this monitor at 1020x1080, which is the default for the monitor. Not sure if DVI would look any better?? No way to tell, as it won't work. I might try it with the new iMAC just for fun, But this problem seems to be kind of intermittent, based on the post to this forum.

Some think the problem is caused by using non-Apple cables. I've had very good success with Monoprice cables and they are about 1/4 the cost of Apple cables. So right now I'm using Monoprice cables for both monitors and they work fine. Wonder if it is a software driver problem in Yosemite??

I Am so flippin ****** off. Iv been using my MacBook as a home theatre media device and this happens to me. My MacBook is reading the display port because the screen flashes but there is still no picture on my hdtv. apole is supposed to be a premium product. They sell their stuff at inflated prices but their **** doesn't ******* work. Iv tried everything including different sequences trying to get my tv to display what's on my Mac. This is what I hate about apple. They have connectivity issues where their stuff can't connect to basically anything then they sell these accessories at inflated prices and later on you find that it doesn't work. Now I wish I had a normal windows notebook. Just ******* plug and play. No ******** like what you get on a Mac!

I have been using Macbook Pro, Samsung HDMI Display, j5create USB3 to HDMI adaptor, without any problem. When I installed Yosemite the Samsung would not work. I followed this suggestion and downloaded the driver form j5create, restarted computer, turned on Samsung, connected it to USB port with adaptor, and IT WORKED!

Im done buying all these apple accessories. The stuff is overpriced and then it stops working.

This 100% worked. For some reason the monitor wouldn't work after the macbook went to sleep. Unplugged monitor power for three minutes and back like a charm.

I have been using my late 2008 13" macbook with various external displays through Mini Display Port to HDMI adapter.

In 2015 it suddenly stopped working with external display. I suspect this is caused by Yosemite OSX but saw that many people since 2011 got similar problems.

Funny thing is, when connect to my Panasonic TV it works with no problem.

But with Philips external display, all i get is NO SIGNAL. This is ridicilous and annoying. It works from time to time (%5 maybe!)

Apple is really in a very wrong way. This is certainly a kind of bug and needed to be fixed already.

Stop ignoring all these people and do something Apple!

OS X Yosemite: If a Thunderbolt device isn’t working

Last Modified: Nov 18, 2014

I have installed an update by apple on 30.06.2015,

One of the updates was "some external displays are not recognized by Macbook" or something like this.

Downloaded and installed, hoping that finally apple heard all these people and fixed it. BUT, nothing changed for me...

Anyone else tried it?