I had previously enabled viewing of system/hidden files for all my folders, and I spent several hours last night looking for large files/folders within Windows Explorer. Searching for files created/modified in the past few days, files over a certain size, etc., unfortunately I didn't find any "smoking gun"
I am a little nervous using diskpart, since a mistake could cause big problems for me, so I am now contemplating what I wanted to avoid, a reformat and reinstallation of Windows. First I will do a complete backup then use diskpart. If unsuccessful with diskpart, I will reformat and reinstall (not a restore) Windows.
I usually prefer to avoid a total reformat/reinstall because even though it may eliminate the issues, it usually masks the underlying cause so I may never know for sure what happened.
I will report my findings back to this thread so it is added to the “knowledge base”. Thank you for all the suggestions.