This video and article will show you how to Fix Error Your bootable USB drive could not be created.

Hi All
When installing Windows on a Mac I came across this error when I was trying to create a USB drive. The problem is the DVD is already mounted in Disk utility and when this is the case the Boot Camp Assistant cannot mount the DVD at the same time so you get this Error Your bootable USB drive could not be created.

The fix for this is pretty straight forward. You need to unmount the USB drive in order to get this to work. But also you will need to make a copy of that DVD so that you will be able to continue along with the setup.
If you want to know how to create a ISO or DMG file from a DVD then you can follow the 123myIT.com video here.
How to create an ISO file on a Mac

Once you have created your DMG or ISO file, you can then start the process again with the Boot Camp Assistant. However this time instead of selecting the DVD drive, go into your Documents folder and select the newly created DMG or ISO file. This should then allow you to continue on with the installation as normal and Fix Error Your bootable USB drive could not be created one and for all.

Users that have upgraded to Yosemite may also get this same error. If this is the case just follow the steps outlined above and you should hopefully resolve the same error.

If you want to see the official Apple page on how to create an ISO image file click on the link below.
Boot Camp: Creating an ISO image from a Windows installation DVD

Good luck
Matt