The Thunderbolt Firmware Update v1.2 was released to Software Update and Apple Support Downloads late last week. It improves the use of Thunderbolt connections for Target Disk Mode, the Support Note suggested.
The Accelerate Your Mac site said there had been some response by readers of problems; mainly, that Thunderbolt devices weren't recognized upon restart. Resetting the System Management Controller appeared to help in those cases.
When installing a firmware update, I try to be very conservative with the process: Of course, a laptop must be plugged in, but I download the update with a wired connection, rather than over wi-fi. And a full backup and clone to an external drive before proceeding is recommended.
I was interested in the symptoms of SMC corruption. It's quite a list and worth rereading:
The computer's fans run at high speed, although the computer is not experiencing heavy usage and is properly ventilated.
The keyboard backlight appears to behave incorrectly (on Mac computers that have this feature).
The Status Indicator Light (SIL) appears to behave incorrectly (on Mac computers that have an SIL).
Battery indicator lights, if present, appear to behave incorrectly (on portables that use non-removable batteries).
The display backlight doesn't respond correctly to ambient light changes on Mac computers that have this feature.
The computer doesn't respond to the power button when pressed.
A portable Mac doesn't appear to respond properly when you close or open the lid.
The computer sleeps or shuts down unexpectedly.
The battery does not appear to be charging properly.
The MagSafe power adaptor LED doesn't appear to indicate the correct activity.
The computer is running unusually slowly, although it is not experiencing abnormally high CPU utilization.
Application icons may "bounce" in the Dock for an extended amount of time when launched.
Applications may not function correctly, or may stop responding after being opened.
A computer that supports target display mode does not switch into or out of target display mode as expected.
A computer that supports target display mode switches into or out of target display mode at unexpected times.
This latest Thunderbolt update reminded me of another recent Support Note, but this one about Windows 8 installations. The symptoms are that users see an alert that Windows couldn't create a new partition or located an existing one during a Windows 8 installation.
So, where would Thunderbolt be in this problem, you might ask? It appears to be something with I/O mapping.
The resolution is to shut down and then unplug any Thunderbolt or USB devices, except for one with the Windows ISO installer.