Apple released an EFI update this week for their new 2013 iMacs – on the very day the hardware was released! The update corrects issues that may occur when installing or using Windows on the new iMacs through Boot Camp, and improves overall support for running Windows 8.
From Apple’s release notes:
iMac (late 2013) EFI Update 2.1
This update is recommended for iMac (late 2013) models. This update addresses an issue which may cause a black screen to appear when installing Windows 7 or Windows 8 using Boot Camp Assistant if both an external optical drive and USB thumb drive are connected to the system. This update also ensures that the system will boot by default into OS X after installation of Windows 8.
The update weighs in at just under 9MB, and is available through Software Update for eligible Macs, as well as direct download from Apple’s support site (link).