Is target display mode using mini-display port working after mountain lion upgrade?

Yes it does. No problems with mine.

It stopped working completely on my late 2009 27" iMac after upgrading to Mountain Lion. I had been using an Atlona dual-link DVI to DisplayPort converter to plug a Windows 7 gaming machine into my iMac without any problem for a very long time. Immediately after upgrading to Mountain Lion, it absolutely doesn't work at all.

My 2010 iMac seems to also be having issues. I usually have my xbox hooked up to play through it using the Belkin AV360, but now it doesn't switch to target display mode at all. 😟

I'm also having target display mode problems with my late 2009 27" iMac. Since upgrading to Mountain Lion, I get no TDM functionality whatsoever. Trying to use my imac as an external monitor with my thinkpad T400s. Used to work pretty well, now nothing. Any solutions out there for this??

Anyone tried to make a lion/snowleopard partitio to see if it works even you upgrade to Mountain Lion???

I installed Lion on a separate partition after upgrading to Mountain Lion and still couldn't get back my previous target display functionality. Maybe somebody else will have better luck?

Hey!

Now i'm trying to downgrade from Mountain Lion to Lion (on the same partition. My first idea was yours, install on a second partition). Do you know how can I downgrade with a USB Lion bootable without loosing my data??? I'll report my results! 🙂

Did anybody find a solution to this? I have the same problem on my late 2009 iMac 27" + Mountain Lion 10.8.4.

Details of my situation here: https://discussions.apple.com/thread/5122980

I'm posting the same response from my other thread, in case it helps you guys...

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I'm able to get target display mode and brightness control working now on Mountain Lion 10.8.4 using my late 2009 iMac.

My solution was to keep hitting cmd+F2 until it works.

It takes me anywhere from 3-15 tries each time, to enter target display mode. The screen will flicker when it is unsuccessful and sometimes just go blank. Sometimes, there is a delay of up to 7 seconds before you see a flicker or blank screen or success. So I might be sitting here for a minute or two just hitting cmd+F2 before it actually works. Very annoying, but at least it's working now.

When I was using Snow Leopard 10.6.8, it always worked on the first try. The cable I'm using is: StarTech Model MDP2DPMM6 6 ft. DisplayPort Adapter Cable M-M.

Switching out of target display mode (cmd+F2 again) works everytime on the first try.

Hope that helps anybody else who had my issue.

Update:

After two days of hitting cmd+F2 over and over, to get target display mode to work, it seemed to get less reliable everytime, until I got tired of trying for 5-10 minutes straight and it still working. I even bought a new, more expensive displayport to mini displayport cable and no luck.

Solution:

I downgraded to 10.7.2 as somebody mentioned in a previous thread and target display mode works and so does brightness control (although the brightness is set to full brightness each time you switch from imac to the other computer--you just lower the brightness each time).

I don't know about any other versions of 10.8.x or 10.7.x, but I can conclude that if you use a Late 2009 iMac, your safest bet is just stick to 10.7.2. Don't put yourself through the torture of 10.8.4.