Hi Jan,
Well, to "boot from USB" was not my objective. I was attempting to create an installation disc on a USB drive as opposed to a DVD. The Windows Media Creation Tool is designed to do this but clearly doesn't work the way I had it in mind. Thus, I'm wasting a huge amount of disk space.
The setup files appear to be hard coded so that it ignores any existing partitions, wipes what's there and then creates it's own before placing the install files on it. All this with no warning to the user. I can't believe I'm the only one who has attempted this.