In the old days of Tech Support we used Target Disk Mode a ton. That usually involved using a Firewire cable between two machines. It worked well, but was a little slow for sure. Now that all things Mac are so connectable Target Disk Mode seems a little antiquated, but don’t bet on it.
Apple eventually expanded the Target Disk Mode from using the Firewire interface and allowed you to use an Ethernet cable. That was because they ended up getting rid of Firewire which was replaced by the new Lightning/Thunderbolt standard (you can read all about Thunderbolt here). The Ethernet solution was an interim measure while Macs transitioned from Firewire ports to Thunderbolt ports. Let me tell you, Ethernet was very slow.
Even though we have all kinds of ways to connect our Macs these days for transferring files (including the cloud) using Target Disk Mode via a Thunderbolt cable is still a good way to go. Honestly, if I was only transferring a small amount of data I would use another method, but for moving large amounts of files and/or using the Migration Assistant to move whole accounts, Target Disk Mode is invaluable.
Target Disk Mode
Most Macs within the last 5 hears have a combo Thunderbolt / Display Port connection. Just on general principles and for a recent major data transfer of my own, I bought a Thunderbolt to Thunderbolt cable (you can buy Thunderbolt to HDMI and to other connectors as well). You can buy knock offs, but I bit the bullet and bought the Apple cable. Needless to say it is excellent quality.
Connecting my 2011 MacBook Air to my Late 2012 iMac was easy. I shut down the Air and connected the two machines with the Thunderbolt cable. Then I started the Air and held down the “T” key until I saw the Thunderbolt symbol on the screen:
I had to take that photo with my iPhone so not a great picture, but you can see the Thunderbolt symbol on the screen. The battery symbol just means the Air is not plugged into AC. After the Target machine is connected you can see it on your Desktop:
You can open the Target drive and move stuff. However, I think I might use Thunderbolt Target Disk Mode more with Migration Assistant in upgrading to a new machine or possibly reinstalling a machine from scratch, YMMV.
Just a note, I did move a 200 MB file from my iMac to the Air as a test. My overall observation? Fast! The transfer was really fast! The Tunderbolt standard is billed as way faster than most anything out there and I believe it.
If you are going to be transferring large files or are using the Migration Assistant app to move to a new machine I highly recommend getting a Thunderbolt cable. It is not a bad idea to just have one on hand really.