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.