Sharing permissions on external hard drive
I have set up for my external hard drive to be shared on the network. When add the folder to share the default permissions are read & write for everyone and guest. I do not want guest connecting to my hard drive through the network and adding files. I want to share with read only permissions. When i change the permissions and close system preferences the system changes the permissions back to default. I have even added my administer account as read & write but that gets removed and the permissions are changed back to default. How do i change the permissions?
MacBook Pro, OS X Mavericks (10.9.2)