Apple could have provided the firmware restore system on a bootable USB flash drive, but instead the firmware recovery process is rolled into the Recovery HD hidden partition that is now included with all new Macs.Even better you can use the following tips to create your own Recovery HD on any volume, including a handy USB flash drive you can carry around with you.When Apple releases Mac firmware updates, the system undergoes a special installation procedure in which it shows a progress bar on a gray screen and then reboots.
Then, if something goes wrong during a firmware update, you can restart your Mac from the Firmware Restoration CD and your Mac will replace the corrupt firmware with the known good version.
There are currently 6 different Firmware Restoration files that cover various Mac models.
Many Intel Macs, that include a built-in CD/DVD drive, have the ability to restore corrupt firmware to a known good state by using a Firmware Restoration CD available from Apple.
(Apple supplies the firmware as a download; you supply the CD.)When Apple removed the CD/DVD drive from Mac models, they realized that an alternate method of recovering from a corrupt firmware installtion was needed.
Mac firmware restoration is the process of resetting your Mac's internal firmware to a known good state.