Mac is unresponsive on startup after installing Thunderbolt Software Update 1.2
You may notice your Mac becomes unresponsive on startup after installing Thunderbolt Software Update 1.2.
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:
Apple has posted an updated version of Thunderbolt Software Update 1.2.1 here.