I am also new to BE, but from what I have seen you could create a rule for offside storage for your media sets. Check the properties for the media set. There you can define a rule when to move tapes to the external storage location and when they could be taken back to your autoloader. But: once your media has been marked for external storage, you need to manually unload these tapes from the autoloader, so that they are physically at the external storage location.