Edit: Solved; somehow a folder from the external drive ended up in the Documents folder instead of Downloads. User-error I've tried searching this and looking through OneDrive settings; there's no setting for the "Downloads" folder. I dragged-and-dropped about 8GB of files from external media to the Downloads folder, and OneDrive is automatically trying to upload all of that. I paused it for now, but I'd like to use OneDrive without this happening.