I am using Windows10Upgrade28085.exe for an upgrade.
My space on the C: drive is limited but I do have ample space on my D: drive (or external storage).
The upgrade program detects the limited space on C: and offers to free up space but not an option to Attach an external storage device.
How do I get this option ? - as descibed in How to Upgrade to Windows 10 Creators Update on a Drive with Limited Space