Apple has posted the MacBook Air Firmware Update 2.7. It’s recommended for all MacBook Air (mid-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. For more info go to http://support.apple.com/kb/DL1689 .