Mac as an external drive
Target Disk Mode is a feature that allows a Mac to act as an external hard drive. Until recently, you enabled Target Disk Mode by connecting two Macs together via FireWire. The new Thunderbolt port on the current iMac and MacBook Pro supports Target Disk Mode, but up until this week, you couldn’t do it because there was no Thunderbolt cable.
Apple has finally released the Thunderbolt cable, so if you happen to have a couple of Thunderbolt Macs, you can enable Target Disk Mode via Thunderbolt.
Remember, this is only possible with 2011 models of the iMac and MacBook Pro (and any other Thunderbolt Macs Apple may release in the future). You’ll also need the Thunderbolt cable, which you can get from Apple for $49. Before you start, update OS X to the latest version and download and install the Thunderbolt firmware update on both of the computers you plan to use.
Step 1: Boot in Target Disk Mode
On the Mac you want to use as the external drive, go to System Preferences > Startup Disk, and click the Target Disk Mode button at the bottom. Click Restart from a drop down menu, and wait for it to reboot.
If the Mac you want to use as the external drive isn’t powered on, hold down the T button on your keyboard while the system boots.
Step 2: Connect via Thunderbolt
Connect one end of the Thunderbolt cable into the Thunderbolt port of the target Mac (the Mac you want to use as the external drive). Then connect the other end to the Thunderbolt port of the host Mac.
Step 3: Thunderbolt Target Disk Mode
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.