If you see both the FireWire and Thunderbolt icons displayed on a gray screen on the target Mac while it boots, it’s now in Target Disk Mode, which should be confirmed when you see the system show up as an external drive on your host computer.
There are two differences when using Target Disk Mode with Thunderbolt instead of FireWire. You’ll see the Thunderbolt icon instead of a FireWire icon flashing on the screen of the target Mac when it is connected to the host. On the host Mac, instead of seeing the target as a FireWire drive, you’ll see a regular hard drive icon.
Other that that, Target Disk Mode works just the same as before, so for those who are familiar with it, you shouldn’t miss a beat.