Apple has posted an EFI update for late 2013 iMacs that address an issue with installing Windows.
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.
You can download the EFI update from the link below or via Software Update on your device.
The new 2013 iMacs feature Intel Haswell processors, new graphics, 802.11ac Wi-Fi and faster PCIe flash storage options.
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