I have a backup drive connected to my mac mini, which I just upgraded to yosemite. I created a guest account, and unfortunately, the guest can browse the backup drive. Is there an easy way to prevent that?
If you use TimeMachine you can encrypt the backup. Go to system preferences, (re-)select the drive and tick the checkbox for encryption. As for the external drive as a whole, you could only apply certain permissions to folders using the information screen (right click the specific folder) or use a drive that supports password protection.