Cloned from: https://blog.bramp.net/post/2008/06/23/create-bootable-windows-server-2008-usb-stick/
Start diskpart
DISKPART> list disk
Select the USB device from the list and substitute the disk number below
when necessary
DISKPART> select disk 1
DISKPART> clean
DISKPART> create partition primary
DISKPART> select partition 1
DISKPART> active
DISKPART> format fs=fat32
DISKPART> assign
DISKPART> exit
xcopy X:\*.* /s/e/f Y:\
where X:\ is your mounted image or physical DVD and Y:\ is your USB
device
Diskpart info: http://www.computerhope.com/diskpart.htm
See also: USB install media with WIM file larger than 4GB