Apple issues Thunderbolt Firmware Update 1.2 for Mac systems
A small update is available to address stability issues involving Target Disk Mode.
Apple has released a firmware update for the Thunderbolt controllers in its Mac systems, which should address problems with using Target Disk Mode to mount a Mac's internal drive on another system as an external drive. Target Disk Mode is commonly used for troubleshooting hard-drive function, or when migrating data to a new Mac.
The update is a 1.22MB download that can be obtained by running the Software Update service in OS X (in the Apple menu), or by downloading it from Apple's Support Downloads Web site and applying it manually.
Though small, the update contains firmware updates for various iMac, MacBook, and Mac Mini systems that include this port. Since Apple does not list specific machine requirements, it should be applicable for any that have a Thunderbolt port.
Be sure when you install the update to follow Apple's onscreen instructions, and make sure the system is attached to a stable power supply. Firmware updating can result in corruption that can render a system useless, though modern firmware updating procedures are relatively secure from corruption in comparison to years past.
The update requires OS X 10.8.3, and will require you to restart your computer in order to install it.