When you install Office software it will create registry items within the Windows folders, which are on the main drive, usually the C drive. So, if you want to be able to run Office on external hard drive, it may fail due to the lack of essential registry contents.
All software registry items only store in system drive. The only way to transfer to external drive only can do after installation complete.
minimal storage? How big is your system drive without anything?
Video location can move to External drive, will save a lot of place.