A data processing system includes a readout unit, a device ID storage unit, a storage medium writing unit, a determination unit, and a processing unit. The readout unit reads out data that have been stored on a storage medium, where the storage medium is removably mounted in an external storage unit. The device ID storage unit stores a device ID that is peculiar to a device. The storage medium writing unit writes the device ID that has been stored in the device ID storage unit to the storage medium. The determination unit makes a determination as to whether the device ID that has been written to the storage medium and the device ID that has been stored in the device ID storage unit are in agreement. The processing unit carries out either first processing or second processing depending on a determination made by the determination unit.