You can install a portable version of any backup / restore program onto the external drive. You can have an icon both on "C drive" and "D drive" that points to said utility. My best guess is that you will start the utility and kick into gear whatever pre-plan you managed to wedge into the backup / restore utility.
Actually, there is a way to eat your cake and have your cake, sort of.
If you make that usb-connected ext HD a boot drive, i.e., from a backup / restore utility ISO such as Macrium Reflect, Image for Windows, EaseUS, AOMEI just to name a few, "burn" a boot onto the ext HD, I think you can tell most UEFIs and almost all BIOS setups to boot this ext HD if it is connected and found. Once that ext HD and the b/r utility has been booted, you simply activate whatever script or pre-plan you set up. Because the "burn is en toto" -- make sure you do that burn before you save anything onto that ext hd; if there is any data already on that ext hd, copy such off onto another device, because again, the "burn is en toto".
Edited by RolandJS, 23 February 2018 - 03:56 PM.