Nautilus action when media inserted

When media is mounted Nautilus will open a new instance to browse this media. This can be disabled in Nautilus menu:

Go to: Edit -> Preferences -> tab 'Media' -> uncheck 'Browse media when inserted'

Note: in this screen you can also configure actions to perform for different media types that are inserted.

Disable 'automount open' with the gconf-editor:

gconf-editor

Go to: apps -> nautilus -> preferences -> uncheck 'media_automount_open'