No sooner had Apple announced the 2013 iMacs than they also issued a day-one EFI (Extensible Firmware Interface) update for the speedy new all-in-ones.
The update is targeted at an issue that sometimes occurs when installing a Boot Camp partition with Windows 7 or Windows 8. The issue appears when an external optical drive (the devices have no internal optical drive) and a USB thumb drive are connected at the same time, causing the screen to go black.
The update also ensures that the default boot drive for the system will be OS X after Windows 8 is installed. iMac (late 2013) EFI Update 2.1 can be downloaded from Apple's support pages, and requires that the Mac be running OS X Mountain Lion 10.8.4 or later.