I needed to make backups of a couple of maintenance USB drives.
It turned out to be pretty easy: How To Backup Bootable USB Drive describes how to do that based on a small USB Image Tool developed in .NET.
One of the things you can do with this is backup bootable Windows installation media.
Note it is a simple tool, so it backups only same size to same size. For more advanced copy purposes, use something like the professional tools from Acronis.
–jeroen