Cannot delete Backup files

I have MyCloud EX2 Ultra connected via my wifi. I use CCC to do backups from my local hard drive. I am trying to delete files on that drive my Finder. When I try, I get a message saying the files are in use. CCC also stalls when trying to “prune” files on this drive. How do I delete files on this drive? Can I connect it directly to my laptop and reformat the drive?