Not sure what you mean.
If you have your library on external, you can disconnect it when you want, unless you have iTunes actually open with that library. Before you launch iTunes make sure you have the drive mounted first.
You can use multiple libraries, for ex. main lib on external and smaller lib on internal hd. You can choose which one you want to load by pressing Option(alt) key after opening iTunes.
Any iTunes media stored on an external drive will not be available if you unplug the drive. The songs will still appear in your iTunes database, but will have exclamation marks (!) next to the songs, indicating they are missing. To make them available, quit iTunes, reconnect the drive, then launch iTunes with the drive mounted. The exclamation marks will be gone and your music will be accessible.If i buy a external Hardrive for all my media library i have and scan it for itunes. If i unpluged it and replug it will still reconize dans library ?