since upgrading to yosemite my MacBook Pro want detect my external display

Hi Dom,

I just looked for updates in the App Store and didn't see any. But I rebooted by computer anyway, then checked again for updates and found none, then I thought maybe the updates were installed without my knowing so I tried plugging in my external monitor again. Still doesn't work.

Is there any way you could give us a link to the update that fixed the problem, or give us some kind of update ID number or description so we can find it too?

Thanks!

P.S. has anyone talked to Apple on the on the telephone or at a Genius Bar about this issue? Just wondering.

My Macbook has no Thunderbolt port, so the update doesn't appear for me. But perhaps there are other components in the update which are part of the issue I'm having with my Mini Display Port. If you look in your /Library/Receipts or ~/Library/Receipts folder you should find the BOM file which was left behind after the installation. Would you mind making a ZIP file of that receipt and posting it here in this thread? (It may not be that easy to find. So you might try the app "Pacifist" and see if it can help.)

What eventually helped me was the PRAM reset, as suggested here Yosemite does not recognize external displays...

OS X Yosemite: Reset your computer’s PRAM

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

PRAM reset 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 plugged cable back in, it worked.

I have the same problem. I'm using Thunderbolt to HDMI connection on ViewSonic from late 2011 Macbook Pro.

About 1 out of 20 tries pulling out the connector and reinserting, the display will connect but I haven't figured out what causes the connection to work. Must be the way I'm sitting in my chair?

I downloaded Yosemite 10.10.1 this week and that's when it became really bad. Failure to connect was only intermittent before that.

The same cable works perfectly to the Viewsonic when I attach my daughter's 2013 Macbook Pro, which has 10.9.5.

I did the repair step last week due to performance issues with 10.10.0.

I do not see a Thunderbolt firmware upgrade available or I would be trying that.

All in all, I've never had such a poor experience with an Apple upgrade.

I tried resetting PRAM with no success, including unplugging and replugging the cable and adapter.

Also tried Disk Utility / Verify and Repair Disk Permissions without success.

Anyone have a fix for this situation yet? Scott Lahteine, any progress on your side?

There was some advice on this thread to "do a clean install of OS X Yosemite". Has anyone done this and if so what were the results?

If successful, how do I "do a clean install of OS X Yosemite"? Do I have to delete the one I'm running on now before I begin? How?

Thanks!

I hoped that the 10.10.1 update would fix this issue, but it only made it worse. Where I used to be able to boot into Mavericks and the MDP-HDMI would work, now HDMI doesn't work in Mavericks either! I have tried all the potential solutions I've seen, both here and elsewhere, including resetting PRAM, resetting SMC, running Disk Utility to fix permissions, etc., but nothing has yet fixed it. My girlfriend has a plastic unibody MacBook, and the HDMI display also stopped working on that machine after the 10.10 update.

I have a MDP-VGA adapter, so I'm now using that to connect to the display's VGA input.

I concur with others here that this is the most shameful and sluggish OS X update yet. Whatever they did to the graphics and event system also makes it slow to respond. Try doing screen zooming using the scroll wheel or trackpad trick on 10.9 and compare to 10.10. Horrible. Even simple things like activating a window typically takes a full second.

Fresh installs are tricky. You have to save the downloaded Yosemite installer (which you can also download at any time from the App Store app). Then you have to make a bootable USB stick - 4GB is too small. There are some instructions posted here.

I did the same steps as wiherek, and it was the second PRAM reset -- with the external monitor disconnected -- that did the trick.

I had already done the permission repair, and I had done a shutdown/restart with the monitor disconnected. So it may not have been ONLY the reset that worked but some combination of the steps I took.

Note that in between the first and second PRAM reset my monitor went fuzzy and complained about the resolution. I doubt I would have tried the second reset without that noticeable indication that something was different.

Now if only the performance issues were resolved...

Good luck everyone.

Well I spoke too soon. My external monitor worked all day yesterday but toward evening it began to flicker. It would go off for a few seconds and then come back on for several minutes. Finally it just went off. Today it won't connect and I don't have time for more of the PRAM nonsense. So my solution wasn't a solution. :-(

I have similar problem since installing Yosemite 10.10 on my 2014 MacBook Pro 15", tried customer support twice and Genius bar. Was told I can not boot my system when using my external monitor as primary display. I have to disconnect monitor, boot system and plug in monitor. NOT happy that I am told this maybe a software issue, wait for an up grade! Is this a problem with others?

At least you can get yours to connect somehow. I've called tech support three times, reset the PRAM, did a clean install, went to the genius bar, got my logic board replaced (as per an Apple "Genius'" suggestion), and I still can't connect my Macbook Pro 15 inch (2012) to my Samsung Series 6 6030.

I've tried everything. At this point, I think we just need to wait for a software patch that will actually make Yosemite work.

I have similar problem since installing Yosemite 10.10 on my 2014 MacBook Pro 15", tried customer support twice and Genius bar. Was told I can not boot my system when using my external monitor as primary display. I have to disconnect monitor, boot system and plug in monitor. NOT happy that I am told this maybe a software issue, wait for an up grade! Is this a pro

SStill no word on how to solve this problem. My PRAM has been reset several times as I mentioned I have been through all suppor channels. Is this what I should come to expect from Apple these days.

*** I'm posting this on all similar questions hoping someone will read it and help everyone out ***

I was having the same problem with Yosemite and thunderbolt to HDMI but have found a sort of a solution or at least a bit more info that might lead to someone having a eureka moment. I have a Mac Book Pro, my partner a Mac Book Air. I bought a thunderbolt to HDMI cable from eBay so my partner could plug her Air into the TV. When the cable arrived, it would not detect our LG TV from either computer. So a bit of playing around and this is what I found.

• I plugged the cable from my Pro direct to the HDMI port on the TV and nothing.
• I also tried from the Pro to the HDMI port on the amplifier of the surround sound system and again nothing.
• However, if I plug the cable from my PRO directly to the TV HDMI and select that HDMI port on the TV initially still nothing. But if I then plug a normal hdmi cable from my amp to the Mac Pro's HDMI port, suddenly the thunderbolt cable works and the Display Preferences see the LGTV. I checked that the correct hdmi port was selected for the direct connection and I could even turn off my amplifier to make sure that I was not seeing the wrong signal and it still worked.

It seems that there needs to be either a base signal or an earthing through the MacBook Pro's HDMI port for it to switch into an external monitor mode that will detect a thunderbolt to HDMI connection. This is fine for my Mac but as my partner's Air only has a thunderbolt port and no HDMI port it still doesn't work with her MAC.

Hopefully someone will see this and have a brainwave.

This solution does not work for what I need. Are you using your LG monitor as the primary display or a secondary? I can start my system with a remote display only if the remote is the slave, not the primary monitor.