Abstract: A method for retrofitting a magnetic computer tape cassette (10) with a theft detection device (12) that emits an audible signal when the cassette is moved adjacent to an apparatus that detects the presence of the detection device, is described. To mount the detection device into the cassette, a groove (70) is cut into the cassette to access a cassette chamber (80) formed between an outer housing (36) that encloses an inner housing (38) that encases the magnetic computer tape (40). The detection device is then inserted into the cassette chamber through the groove and held wedged in place by a V-shaped spring (76). The groove in the cassette is finally sealed with a strip of pressure sensitive tape (88). This method of retrofitting the cassette with the theft detection device ensures that the detection device cannot be removed from the magnetic computer tape cassette without visibly damaging the cassette.