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