Update (Sept 2017) –
An even easier way to do this is to copy your wim image to a windows 10 computer and run the “Windows to Go” application. In the Windows to Go app, choose the disk you want to install the image on and then choose the wim image you want to put on that disk.
The result should be a bootable windows 10 external disk that will boot on just about any computer!
Using a Windows 10 computer, follow the standard procedure to format the external drive and set it to be bootable in DiskPart:
Diskpart
Diskpart> list disk
Diskpart> sel disk 1 (be sure to select the correct disk!)
Diskpart> clean
Diskpart> create partition primary
Diskpart> format fs=NTSF quick
Diskpart> active
Diskpart> assign letter=g
Diskpart> exit
Then with the Windows install disk in drive D, copy and apply the install image to the new disk:
(the “/appy-image” option for dism seems to be available only in windows 10. If the CD isn’t in the D drive change the imagefile option to point to the correct path to the install.wim file on the cd))
dism /apply-image /imagefile=D:\sources\install.wim /index:1 /applydir=g:\
Finally, make sure the drive is bootable:
(id you didn’t assign letter g, then change accordingly)
bcdboot g:\windows /s g: /f ALL