Chosen Solution
I’m trying to use an old iMac 27-inch (Mid 2010) as a monitor, using the Target Display Mode without success. I’m connecting a MacBook Pro (Retina, 15-inch, Mid 2015). The cable is a completely new Thunderbolt - Thunderbolt cable. I’ve tried resetting SMC and NVRAM on the iMac which haven’t solved the problem. I’ve also tried the VirtualKVM application but it looks as if the Thunderbolt port isn’t recognising the cable. Obviously the CMD+F2 option isn’t working either. Any ideas?
You’ll need to get a DisplayPort cable Vs a Thunderbolt cable. Surprising even though they look the same they don’t work the same! Your iMac has a mini DisplayPort not Thunderbolt so it can’t power the cable which is needed for the head-end circuitry located in the cables connector.
Dear ladies and gentlemen, It’s not the problem with the cable or the adapters, I have a Mac Mini 2018 running Mojave and an 27” iMac mid 2010 running High Sierra. I used a USB-C to mini DP cable. When I connected them it worked perfectly command f2 button was pressed only once, after when I was plugging the cable into my Mac it was detecting the Mac Mini automatically. But, here is the big but that screwed up all the users of this amazing feature , macOS Catalina, and the latest update for older versions of Combined OS Update, its not the problem in the cables or adapters because the TDM was not working directly after I UPDATED my both Macs, its a waste of time and money if you want to buy different cables, the combined update is something like Catalina only for the older models. So good luck to everyone … Check you version of MacOS and prove me if Im wrong!!!! Target Display Mode is not working with macOS Catalina
