No backlight in Target Display Mode on 2012 27" iMac

I am trying to use my new 2012 iMac 27" as an external display for my mid 2011 Mac Mini with Target Display Mode.  At first it seems like it's not working when switching as I hit Command + F2 and the screen changes to black.  But after shining a flashlight onto the monitor I can see the desktop and dock of my Mac Mini operating system.  Mouse control and everything else works, it's just the backlight doesn't light up when connected to the Mac Mini.  But hitting Command + F2 again brings it back to the iMac and a normal functioning backlight.
I'm assuming this is a bug in the new iMac because I had a late 2011 iMac 27" (previous generation) hooked up to this same Mac Mini and Target Display Mode worked perfectly between the two.  It was even the same exact Thunderbolt cable.  I have re-tested the Mac Mini and plugged it into another monitor with a DVI to Mini Display port adapter and it has worked fine.  Has anybody else experienced this behavior?

HI - i had this problem too.  see my previous post above.  I spent weeks on the phone to apple support. 
from memory:
1. I had to reinstall the OS (from a download) - but this failed to work
2. So... I had to perform a complete reinstall of the operating system (and wipe all user data)
3. and then restore all my user data. 
Everything has worked seamlessly since then - which to me indicates it's a s/w problem and possibly one with something in my user profile?  I just reliased I did not try and create a new user and try TDM using that.  This would've proved it was a user profile problem or not..  I had one small ocassion where the screen went blank a week ago, but I just quickly pulled the TB cable out and put it back in again - and it was all ok.
So in essence I've been runing OK now since I did the reinstall and restored the user data.
Richard

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