Late 2009 iMac + 10.8.4 + Target Display Mode + Brightness Control?

I just wanted to confirm that you're not alone with this issue. I have a mid-2010 27" iMac and having the exact same issues. I've used this setup for over 2 years without problems but after 10.8.4, I have to spend 5-15mins pressing cmd+F2 every time I want to switch to TDM 😟

I started my own thread earlier (https://discussions.apple.com/thread/5085224?start=0&tstart=0) but the suggestion to repair security didn't help. I've since tried reinstalling OS X to no avail.

I'm seriously considering selling the iMac and buying a decent monitor instead.

To keep hitting command-F2 until it finally works is not really a solution, is it?

I'm having the exact same problems since updating to 10.8.4, and it's been driving me insane after troubleshooting for days now, and only now finding this thread here. It takes me 10+ tries of hitting command-F2 until it finally works, and it's maddening. I don't even dare put the computers to sleep anymore to not have to go through it again.

I hope there'll be a fix from Apple very soon (10.8.5?) as otherwise I'll have to completely reinstall the OS after wiping the HD first.

This has definitely burned me from ever auto-updating the OS again.

I'm disabling all auto-updates now. I truly hope Apple is listening and will fix this very soon.

I'm having the exact same problem with 10.8.4 running on my 27" Late 2009 iMac. I have my iMac hooked up to my PC's Radeon 6870 via DisplayPort. It has always worked perfectly for me, but now the screen either just flickers, or goes to a black screen. If you keep trying, eventually it will switch successfully, but it takes a LOT of tries. I hope Apple fixes this soon, or someone finds a better workaround. This is a very important feature for me. I built my PC around it and it heavily influenced my decision to buy the iMac itself.

Wow, am I lucky that I found this thread as I was updating from 10.8.2 to 10.8.4! I force quit the App Store straight away as it doesn't let you stop downloading an update (not installing, but just downloading!) without force quitting the App Store.

In the past there have been a few breaks to the target display mode in some updates. It's an absolute shambles how Apple releases updates that time to time again, breaks target display mode functionality. For me, target display mode was the reason for buying an iMac, knowing that I will always be able to use it as a monitor for my MBP 15" and also my gaming PC.

10.6.x (what ever shipped with the 2010 27" iMac) was great. Updating to 10.6.8 broke being able to adjust brightness when in target display mode, it was always 100%. That was really annoying and made me upgrade to Mountain Lion (after reading the forums that ML returned functionality that Apple broke with their update).

10.8.1 then was great, everything worked, it was a mistake going to 10.8.2.

Since updating to 10.8.2 the internal speakers have really bad distortion for about 10 seconds after not having any audio played through them for a few minutes. That's ok, I can live with that, but there was an even worse bug that 10.8.2 brought to target display mode. Once in about 10 times of using cmd-f2 to switch back to my iMac from my gaming PC, the iMac stops responding to my USB keyboard + mouse, and also to the bluetooth mouse and keyboard. It's a real PITA to have to hold the power key down and wait the long start up time from a forced shutdown once a day. It's an OS software issue as I am still able to use Teamviewer to remote control the iMac as normal, just neither the bluetooth or USB inputs will work.

I was going to update to 10.8.4 but after reading that there's even worse issues, i'll hold back and put up with the pain that is 10.8.2, as 10.8.4 sounds absoleutely unuseable for my purpose.

Apple please get your act together and TEST your updates with iMac's equiped with target display mode BEFORE releasing the updates. I'll be happy to put up my hand to test updates as I am a heavy user of target display mode with multiple devices.

As is, it is a bad reflection of your company that users have to put up with your broken updates, and that you don't learn from your mistakes time and time again.

Shame really.

Hey guys I found this fix using a free 3rd party app called Shade see article here that suggested this: http://www.iosappadvice.com/forum/how-to-adjust-imac-screen-brightness-in-target -display-mode/

works pretty well and is free (you can donate to the guy though, I may if I end up using this Imac as an external like this) Hope this helps someone

When the HDD on my late 2009 27" iMac gave up the ghost for the second time I decided to try to use the screen of the iMac as output for a new Mac Mini and/or as a second screen for my 13" Macbook Pro. I tried all the usual things mentioned in the discussion groups but kept on getting a either a black screen or just a very faint image. A few had suggested using a Belkin mini display port cable since Apple no longer produce them. I want to state loud and clearly that this turned out to be extremely important and made a night and day difference. Up to then I had been trying a Brazilian produced mini display port cable. In fact the image is now better than the original iMac and is absolutely stunning, possibly because the image board in the Mac Mini is better than the one in the 2009 iMac. Apple equipment in Brazil is at least two to three times as expensive as in the US because of huge importation taxes and this has save me at least $2000 to buy a quality screen. Strangely, I do not need to carry out the CMD-F2 keystroke to activate the connection, it just connects directly.

Peter Pearson, Sao Paulo, Brazil