If you have a Mac that doesn’t have an optical drive, Apple has modified the Boot Camp Assistant to help create a USB stick that can boot up the Windows 8.1 (or Windows 7) installer. This option is only available on select MacOS X computers– mostly ones without a USB drive; however, you can modify Boot Camp Assistant to help create this installer on almost any computer.
0. Find info.plist configuration file in the your bootcamp package.
1. Add your Boot Rom Version(from system info) under DARequiredROMVersions section.
2. Add your Mac’s Model Identifier (from system info) under PreUSBBootSupportedModels section
3. Delete the word “Pre” from “PreUSBBootSupportedModels”, so you have “USBBootSupportedModels”
The next step is only for MacOS X 10.9 and above, as these newer MacOS X versions employs code signature verification, to prevent you from changing info.plist and cause bootcamp to crash.
4. Open your terminal, use the following command
sudo codesign -fs - /Applications/Utilities/Boot\ Camp\ Assistant.app