Apple 27-inch iMac EFI FW Update...The update is recommended for all quad-core Intel Core i5 and Core i7 processors 27-inch iMacs

This update addresses the following:

• Resolves an issue that sometimes caused high processor utilization while playing audio through the headphone output mini-jack.
• Resolves an issue that prevented the display backlight from turning on after powering on the iMac.

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 iMac during this update