I recently started having some problems with my Windows 7 installation on my mid-2010 iMac so decided it was time for a refresh and full update.
I backed up my files, booted in to OS X, repartitioned the drive, updated to Yosemite and proceeded to run the Bootcamp Assistant to install Windows 8.1 (with update).
That's when I noticed a problem. I was not given the option to create a bootable USB installer. While my iMac does contain a SuperDrive I don't really have any DVD-Rs lying round these days so wanted to use one of my fast USB sticks to install Windows from.
Luckily, this is easy to remedy. The only thing stopping Bootcamp Assistant from allowing me to create a USB installer was a flag in the application's settings. This small section from the Info.plist to be precise:
We can edit this plist, which then stops Bootcamp Assistant from hiding the USB installation option! All you need to do is load up Terminal (Launchpad->Other->Terminal) and enter the following commands:
sudo sed -i .old 's/PreUSB/USB/g' /Applications/Utilities/Boot\ Camp\ Assistant.app/Contents/Info.plist
This will replace the key 'PreUSBBootSupportedModels' with 'USBBootSupportedModels'. Note that you will be prompted for your password, you'll require root access to write in to this file.
sudo codesign -fs - /Applications/Utilities/Boot\ Camp\ Assistant.app/
This will codesign the modified file (to make OSX security settings happy). You may again be prompted for your password, and you may be asked to install the command line developer tools (if you don't have XCode installed). If you are asked to install the tools, then install them and re-enter the command.
Now, load up Bootcamp Assistant and you should see a new checkbox allowing you to create a new USB installer for Windows:
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