Protecting folders from deletion while allowing file addition

I have an external drive comprising a folder, containing multiple levels of folders, that I would like to protect from accidental deletion or from files being moved between folders. If I set the permission of the folder (and to the enclosed items) to read only I can no longer add files into the folder or create new subfolders in the folder. I would like to set up the permissions (probably ACL) to prevent the folders and files from deletion while allowing addition of new files and subfolders. Can anyone help me ? Thanks