You will notice that external drives are usually shown with an orange icon.
If your Mac has a software problem, booting from an external drive usually works. In addition to using another Mac to boot a system that is unresponsive in target drive mode, you can also use a working system on an external hard drive. Target Drive Mode allows you to transfer files directly from your target Mac to your Mac host as if your target Mac were just an external hard drive attached to your Mac host. Using an external hard drive as a target Mac
Problems can occur for any of the following reasons: It’s impossible to tell exactly why Target Disk Mode isn’t working on your Mac computer.