Resolution

Apple has identified an issue with Thunderbolt Update 1.2 (released Monday, June 11th, 2012) which caused some Macs to become unresponsive on startup after the update was installed. Apple promptly withdrew the update and investigated, discovering an installation compatibility issue with a previous Thunderbolt update applied by some customers. Apple apologizes for the disruption this caused for customers with affected Macs.

Thunderbolt Software Update 1.2 enables Mac computers to support the Thunderbolt to Gigabit Ethernet Adapter. If you do not plan to use this adapter, you do not need to install the update.

To recover a Mac computer that will not start up after installing Thunderbolt Software Update 1.2, start up in Lion Recovery by pressing and holding the Command and R keys after hearing the start up chime. Reinstall Mac OS X. Reinstalling Mac OS X using Lion Recovery does not remove any personal data or files from your Mac.

Alternatively, you may perform either of these options:

1. Restore from a Time Machine backup that was created before Thunderbolt Software Update 1.2 was installed.
2. If you have access to another Mac, you can start up the affected Mac in Target Disk Mode and then install the OS X Lion v10.7.4 Combo Update to resolve this issue quickly.