Apple has released Thunderbolt Firmware Update v1.2 to address stability issues with using Target Disk Mode (which makes one Mac appear as a hard drive to another Thunderbolt-connected Mac). The update is for only those Macs with a Thunderbolt port, and it requires OS X 10.8.3 Mountain Lion or later. As always with firmware updates, although you can download it manually, we recommend relying on Software Update or the App Store app to ensure you get the right firmware update for your Thunderbolt-equipped Mac, and be sure not to interrupt the update process. (Free, 1.22 MB)
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