A security device for attachment to a container having external walls such that a transponder thereon is located externally of or adjacent to one of said external walls so as to avoid being shielded by a metal layer within the container (e.g. the metal layer within a disk such as a CD or DVD). The security device is arranged to trigger an alarm signal if the container is moved within the range of an alarm unit with the security device attached thereto, and has a releasable locking mechanism to secure it to the container such that release of the locking mechanism requires the use of authorized release means. In one embodiments the transponder is located in the vicinity of a window in a metal layer, e.g. at the centre of the disk.