I have an external USB drive that is formatted for Max OX Extended (Journaled). It has a backup of my music collection so I want to write protect it so that it can only be READ and not erased or any of the files erased. How can this be accomplished - running an iMac and El Capitan. I have tried right clicking on the drive and the sub folders in Finder and changing the Read/Write attributes to READ ONLY for myself and everyone, but I can still delete individual files. Thanks.