A data protection method is used for protecting data recorded on a removable storage medium, such as an optical magnetic disk. A conductive end of the cable inserted into the terminal box through the first or second cable hole, the conductive end of the cable is pressed by the pressing portions of the plurality of steel elastic sheets and tightly contacts the arc slot of the conductive column and is secured by the one-way securing portions of the plurality of steel elastic sheets. The device for transforming the high-frequency electrical signals supplied by the photoelectric receiver into low-frequency measuring signals can include at least one switch whose switching frequency is controlled by a control frequency whose frequency is higher or lower than the modulation frequency by the amount of the low-frequency measuring signal.