Thank you for your informative post. I appreciate the response.
I eventually worked out that I could see the file segments that I uploaded and that I couldn't see the actual content. I finished up going with Mountain Duck. (Albeit after purchasing a copy of Air File Drive at a good discount and then finding that I couldn't see the content that way either because I ignorantly had a checkbox unselected in the options dialog.)
I thus achieved my ultimate goal of being able to transfer my files (photos) from OneDrive to SmugMug without having them sit on my hard drive (other than in the cache).