Apple announced a new iMac this morning, and now a software update has already been pushed out for the new hardware.
EFI Update 2.1 was released today for Apple’s late 2013 iMac. It “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.” The update “also ensures that the system will boot by default into OS X after installation of Windows 8.”
Source: Apple
