Using iMac in Target Display Mode with a MacBook Pro

by John Arrowwood   Last Updated August 13, 2019 16:12 PM - source

I'd like to use my old iMac (27-inch, Late 2012) as an external display for my old MacBook Pro (13-inch, Mid 2009).

The hard drive in the iMac is failing, so until I get it replaced, I've installed MacOS Mojave to a thumb drive. I'm able to boot the iMac, no problem.

However, when I connect the iMac to the MacBook Pro and press Command-F2, nothing happens.

I know the iMac supports Target Display Mode, because I used to use it for that when connecting to my old work laptop.

It's the same cable I used to use, so it's not a cable problem.

I've successfully driven my LG monitor using the Mini Display Port on the MacBook Pro, so I'm pretty sure the port is fully operational.

What could be stopping it from working?

Is there anything about booting Mojave from an external flash drive that might stop Target Display Mode from working?

Or is it possible that the MacBook Pro just can't handle driving that particular display?

Any suggestions on how to get this working?

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Updated November 28, 2018 19:12 PM