Today Apple released EFI firmware update 2.7 for the 2013 MacBook Air that addresses an issue with installing Windows in Boot Camp. The update is recommended for all MacBook Airs released after Apple’s hardware refresh over the summer at WWDC. Grab it now in the Mac App Store.
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.
- Source Apple