My brother has ten corrupted Excel files on a backup drive labeled M:'>

Windows Explorer will not let him delete the corrupted files. We checked to make sure no programs were running in the background, i.e. Excel. I asked him to send one of the files as an attachment after making sure there was no virus. However, email would not let him attach any of the files saying: "The file does not exist".

Why do the files show up in Explorer and can not be deleted, while at the same time the files don't exist?