Apple has released Thunderbolt Firmware Update 1.2 (http://tinyurl.com/ctyjsoe). It provides stability fixes for Thunderbolt and Target Disk Mode.
It’s a 1.22MB download and requires Mac OS X 10.8.3 or higher. When your computer restarts a gray screen will appear with a status bar to indicate the progress of the update. Don’t disturb or shut off the power on your computer during this update. Note: Your computer must be plugged into AC power for the update to install.