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Joe asked: “I got a brand new iMac 27-inch in 2017. It’s replacing a 2011 iMac, 27-inch. I was wanting to use the old iMac as a screen to set up beside my regular iMac to do dual-screen things. Is there a way to do that? I have never seen a cable that goes from Thunderbolt to Lightning. I really hate to just put this old computer to bed.”
Joe, you should be able to do it very easily. Working as a secondary monitor was actually a feature on iMacs from 2009 until 2014, there was actually some backlash to the feature disappearing on 4k and 5k iMacs.
Your new computer has Thunderbolt 3 over a USB-C connector, and your old computer has the old, squarish Thunderbolt 2 plug. You’ll need a Thunderbolt 3 to Thunderbolt 2 adapter, which you can buy from Apple for about $50.
Working as a secondary monitor was actually a feature on iMacs
After you connect the computers with it, turn them both on, and on the older iMac that you want to use as an external monitor hold down the Command key and F2.
That should put you in what Apple calls “Target Display Mode” and it is actually hardware based, so there should be no delay, it should work as a legitimate external monitor.
If you need to move files from the old computer to the new one, Macs also support something called Target Disk Mode, hold down T when the old computer is booting, that should give you the option to put it in Target Disk Mode, once you do that it’ll act as an external drive for your newer computer allowing you to move or access files easily.
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