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Apple Thunderbolt 1.2 firmware update not installing on MAC OS X

This is a common problem when updating critical firmware updates for Mac systems. A simple solution to problem is shown below.
The question is:
Are you updating firmware connected to a power source?
If you are not, this is common. MAC OS X will not update ANYTHING critical as (any) firmware if it’s not connected to an external power source (Magsafe power plug). Battery is not considered by MAC OS X as a reliable power supply, therefore it will prevent ANY failure during such a critical process (that must not be interrupted in any way) by not allowing it at all.
You will experience the following:
– Update will download.
– Update will uncompress and start installing
– When done installing update it will reboot.
– A gray screen will appear, then the Apple logo will show in darker gray.
– A status bar will show up, eventually for about 1 second (that is the time the EFI delays to know if it’s magsafe is connected or not).
– Then the process will be interrupted by itself as a safety feature in the moment it notices it is not connected.
So next time do it normally, and make sure your Magsafe power plug is connected to your Mac before you start updating.
Credit goes to MacRumors for this answer.
The update requires OS X 10.8.3, and will require you to restart your computer in order to install it.
About Thunderbolt Firmware Update v1.2
This update provides stability fixes for Thunderbolt and Target Disk Mode.
When your computer restarts a gray screen will appear with a status bar to indicate the progress of the update.
Do not disturb or shut off the power on your computer during this update. Note: Your computer must be plugged into AC power for the update to install.
Hope this helps someone.