Hello, Millicent here.
I don't know if you ever fixed the issue, but something that helped me was following the path from my C: drive to the riot client folder. There should be a file named "RiotClientServices.exe.old". Simply remove the ".old" from the file by renaming it, then go into your Settings > Apps > Programs and Features, then uninstall the Riot Client. That's what worked for me, and I got my storage back.
Have a lovely day!