MacOS – Target Display Mode between two MacBook Pro

yeah, toggling the function key setting worked for me. I'm on 17" 2011 mbp and the 27 inch cinema display with Lion.

I would not recommend it since this may cause issues. The installation process of OS X is specific to the underlying hardware. So if you just clone the iMac HDD and use it in a MacBook Pro, it's possible that you may encounter issues.

It won't hurt to try it out, so do try. But when you try it:

1. Make sure that you press Cmd+V when you hear the startup chime on the MacBook Pro and hold it until you see a Verbose Boot initiated (with many Unix related messages coming up on the text screen and scrolling up). You can read through the messages to see if any serious looking errors appear before the login window comes up.

2. Also note how long the startup process takes and whether it seems slower than it should be.

3. After you're logged in, you can also check Console.app for errors during startup.

For a good experience, I would instead recommend that you do a fresh install on the new HDD (with the HDD installed on the MacBook Pro) and then use Migration Assistant to copy applications and files over.