A data processor is communicable with a host device via a network. The data processor includes a media connecting unit, a file creating unit, a data transmitting unit, and an operation executing unit. The media connecting unit is capable of connecting a storage medium. The file creating unit creates a controlling file for controlling the storage medium. The data transmitting unit transmits data identifying the controlling file to the host device for allowing the host device to access the file. The operation executing unit executes an operation of the storage medium in response to receiving a transmission request of the controlling file.