A data processing system having a host computer including a key manager, a control unit connected to the host computer, a data storage unit (such as a tape drive) controlled by the control unit, and data storage medium for storing data thereon to be written to or read from by the data storage unit. The key manager stores a data structure having at least one record having a volume serial number, as start location, a length entry, and a key for encrypting and decrypting data on the data storage medium. A data storage medium (such as data tape) is mounted on the data storage unit, and a volume recorded on the tape is retrieved. The control unit retrieves the data structure from the key manager and matches the volume serial number recorded in the retrieved data structure with the volume serial number retrieved from the data storage medium. It they match, the control unit passes to the data storage unit, commands to turn on or turn off encryption dependent upon the location where data is written by the data storage unit onto the data storage medium, or to turn on or turn off decryption dependent upon the location where data is read by the data storage unit from the data storage medium.