Applies to MacOS 10.10 Yosemite, it may also apple to MacOS 10.11 El Capitan
It seems Bootcamp Assistant can be a little reluctant to prepare a USB stick to make a Windows 7, 8 or 10 installer.
Anyway, after a bit of research here’s how.
You need an 8Gb USB stick (a 4Gb might do it depending on your installer)
A .iso with the Windows installer. 8.1 is freely available, 7 is a pay for version.
1. Copy Bootcamp Assistant from Applications>Utilities to the Desktop
2. Press File>Get Info and make it read/write for admin
3. Right-click, choose Show Package Contents
4. Open info.plist preferable in XCode but in any text editor if not. The following instructions apply to XCode but if you edit the XML directly it should be fairly obvious how to do it. If not are you sure you want to do this…. 😉
5. goto Apple Menu>(alt)System Information and copy the Boot ROM Version, make a note of your Model Identifier
6. goto DARequiredROMVersions in info.plist and create a new entry, paste the Boot ROM Version
7. goto PreUSBBootSupportedModels change it to USBBootSupportedModels, make sure your Model Identifier is in the list, add it if not.
8. Save and Close info.plist
9. Open Terminal, type sudo codesign -fs – drag your Bootcamp Assistant to the window so it now reads sudo codesign -fs – /Volumes/[whereever]/[whoever]/Desktop/Boot\ Camp\ Assistant.app press return.
10. Run Bootcamp Assistant.
11. There will now be the option Create a Windows 7 or later version install disk
Note: When you choose this option and run, it will appear to freeze at about 7/8 of the progress. It almost certainly hasn’t, just leave it, it’s copying the .iso without telling you and that takes a while. When I did this, it failed a couple of times first and I had to persist.