iMac 5K Target Display Mode (TDM) Solution
I just purchased a new 27" iMac 5K and was excited to use it until i got home and found out that TDM is not supported on the new iMac. it is so important to me that i considered returning it. i have a Macbook Air that i have used with a thunderbolt cable to view it on my 27" iMac as i need access to both computers all day. i thought i was a rarity but come to find out many of you are on the same boat and from what I'm reading many people are either holding off purchasing an iMac or returning theirs because TDM is that important.
Anyway the solution is really simple and it will cost you 20 bucks, but well worth it. I actually like this better because now i can still use the same keyboard and mouse.
Step 1. Download SwitchresX on your laptop http://www.madrau.com/srx_download/download.html
Step 2. Change the resolution on laptop to 2560 X 1440
Step 3. Connect a thunderbolt from your iMac to your laptop. (this should immediately show your Macbook Under shared in finder)
Step 4. click on Share Screen
Step 5. login with your laptop username and password.
there is very little to no lag and this is better for me than 2 keyboards and 2 mice and having to switch using Command+F2. you can try my method free with a 10 day trial for switchesx
iMac, Mac OS X (10.7.2), null