an article surveillance marker suitable for use on computer magnetic data disks is disclosed in which the marker , a length of low coercivity , high magnetic permeability ferromagnetic material is integrally formed onto the data disk preferably so as to serve as the concentricity determining factor of the disk , whereby attempted removal or override of the marker renders the disk from unauthorized reproduction . a disk drive sytem is also disclosed with a built - in interrogation zone and write inhibit circuit that insures data originating from the host computer or copied from another marker - protected disk can only be written onto a disk having a marker - protected disk .