Apple yesterday released Thunderbolt Software Update 1.2, with the single comment that it “Adds support for the Apple Thunderbolt to Gigabit Ethernet Adapter.” Although it’s unclear if all Thunderbolt-equipped Macs are affected, a number of people have reported that installing the update causes a variety of strange boot-related problems (kernel panics, getting stuck at the gray boot screen, “Unexpected error” messages, and more). Regardless of the details, in each case, it seems that the Mac is rendered unusable, even thunderstruck. I’ve now heard from some people for whom the update did not cause any problems, but it’s still unclear what the difference may be between those who are and are not suffering boot failures.
In all the reports I’ve seen, reinstalling Lion fixes the problem. Boot into Recovery Mode by restarting while holding down Command-R. Once booted from Lion’s hidden Recovery HD volume, you can select Reinstall Mac OS X (if there is no Recovery HD volume, or it’s damaged, most Mac models introduced since 2010 support Lion Internet Recovery, which enables them to download a new Recovery HD image and boot from that). Of course, redownloading Lion from the Mac App Store will take quite some time, depending on the speed of your Internet connection. Once you have reinstalled Lion, you’ll need to run Software Update to bring everything up to date; don’t reinstall the Thunderbolt Software Update 1.2 again, or the problem will recur.
If you happen to have a second Mac available, you can also reportedly recover by reinstalling just the Mac OS X 10.7.4 Combo Update. To do this, restart the thunderstruck Mac while holding down the T key to put it into Target Drive Mode. Then connect it to your working Mac via FireWire or Thunderbolt, start the update, and select the thunderstruck Mac’s hard drive at the appropriate point in the installer. For more troubleshooting techniques and recovery details, see “Take Control of Troubleshooting Your Mac, Second Edition.”
I believe Apple is aware of the problem, since a number of people have reported it to AppleCare. Thus, it’s likely that an update will appear soon; either way, unless you absolutely need that Thunderbolt to Gigabit Ethernet Adapter (which works only with the just-released MacBook Air with Thunderbolt and the MacBook Pro with Retina Display anyway), there’s no reason to install this update until we’ve seen a fix.
Thanks to alert reader Tom Barry for the heads-up!