A computer program product, according to one embodiment, includes a computer readable storage medium having program instructions embodied therewith, the program instructions executable by a controller to cause the controller to: monitor, by the controller, a resistance value of each of a plurality of detector structures positioned in an array, and detect, by the controller, a change in a resistance value of at least one of the detector structures, for identifying a defect on a magnetic medium. Each of the detector structures includes a pair of conductive layers separated by an insulating material. Moreover, none of the detector structures include an operable reader for reading data from a magnetic medium. Other systems, methods, and computer program products are described in additional embodiments.