Apple has released a new update for 2012 MacBook Pro systems that fixes problems with the handling of bus-powered Thunderbolt devices.
Thunderbolt is the next-generation I/O technology that Apple is implementing in its Mac systems, which allows very high-bandwidth communication between devices, and also allows for expansion of the PCIe (PCI Express) bus as well as carrying the DisplayPort signal for external monitors.
As Thunderbolt is relatively new, some bugs are bound to crop up in various implementations, and with the MacBook Pro systems produced in mid-2012 it's been found that some bus-powered Thunderbolt devices may not work properly.
To tackle this, Apple has issued a small firmware update that should allow bus-powered devices to communicate properly with these systems. The update is a small 442KB download that should be available through the Mac App Store's Software Update service (available by selecting Software Update in the Apple menu). Alternatively, you can download the updater from Apple's Firmware Update v1.1 Web page and install it manually.
The update requires OS X 10.7.4 or later to install, and will require a restart of the computer during which you should be sure it is connected to a reliable power source and not interrupted during the installation process.
While Software Update should determine if your system qualifies for the update, you can also determine this by choosing "About this Mac" from the Apple menu and then clicking "More Info..." to see the model number of your Mac. If you see "Mid 2012" listed under the type of Mac it is, then the update applies to your system. Alternatively, you can download the updater and run it, and if your system needs it then it will continue and install, whereas if not then the installer will alert you that your system does not qualify.