In addition to the MacBook, MacBook Pro, and MacBook Air EFI updates, the iMac EFI firmware was also updated to version 1.3. Apple says that this update "fixes several issues to improve the stability of iMac computers."
You can download this update by opening Software Update (Apple menu > Software Update) or by downloading the installer package from the Apple's support website.
Please note: To complete the firmware update process, please follow the instructions in the updater application (/Applications/Utilities/iMac EFI Firmware Update.app). The updater will launch automatically when the Installer closes.
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