If you get an error on your Mac about ‘External Hard Drive Read Only’ and are not able to access or copy your files and folders stored on it, then this means there is an issue with the file system. There are many different file systems like FAT, NTFS, APFS etc. designed to store your data on the external hard disk.  https://www.bestmaccleaner.com/how-to-fix-read-only-external-drive-problems-on-your-mac/