I have found a workaround for creating a USB boot device in El Capitan. With El Capitan permissions are locked and there is no way to edit the info.plist file. Here are the steps to correct this so you can create a bootable USB using bootcamp.
Go to Applications and then Utilities and then Right Click on Boot Camp Assistant and Duplicate it. Now you should have one that says Boot Camp Assistant Copy. Right click the Copy and click Show Package Contents, now locate the info.plist file and open it with text edit. Find
<key>PreUSBBootSupportedModels</key>
<array>
<string>MacBook7,1</string>
<string>MacBookAir3,2</string>
<string>MacBookPro8.1</string>
<string>MacBookPro8,3</string>
<string>MacPro5,1</string>
<string>Macmini4,1</string>
<string>iMac12,2</string>
Make sure you remove Pre from in front of USBBootSupportedModels and go to your apple logo click About this Mac, System Support and see your Boot Rom Version and make sure that you add it to the list. Once added to the list then you can close the file and thats it.
When you go to use Boot Camp Assistant just use the one that you duplicated.
Go to Applications and then Utilities and then Right Click on Boot Camp Assistant and Duplicate it. Now you should have one that says Boot Camp Assistant Copy. Right click the Copy and click Show Package Contents, now locate the info.plist file and open it with text edit. Find
<key>PreUSBBootSupportedModels</key>
<array>
<string>MacBook7,1</string>
<string>MacBookAir3,2</string>
<string>MacBookPro8.1</string>
<string>MacBookPro8,3</string>
<string>MacPro5,1</string>
<string>Macmini4,1</string>
<string>iMac12,2</string>
Make sure you remove Pre from in front of USBBootSupportedModels and go to your apple logo click About this Mac, System Support and see your Boot Rom Version and make sure that you add it to the list. Once added to the list then you can close the file and thats it.
When you go to use Boot Camp Assistant just use the one that you duplicated.