A window will appear with all the information about the Mac you are using, including the model name, serial number, memory, and processor. To check your iMac version, click the Apple logo at the topmost left of the screen, then click About this Mac.
Unfortunately, iMac Retina 5K, late 2014, and newer models can’t be used as external displays because the Thunderbolt 2 cable being used for TDM cannot support the new ultra high-resolution display. Here’s a list of the versions capable of Target Display Mode: This means that only versions released after the 2009 iMac have TDM support. Target Display Mode was introduced with the release of the 27” 2009 iMacs installed with Mac OS X 10.6.1. Not all iMacs can be used as an external display. Method 1: Check if Your iMac Supports Target Display Mode. Check out the solutions outlined below to figure out how to fix Target Display Mode. Knowing the cause of the problem makes troubleshooting easier.