A technique to determine whether a memory unit currently attached to a data processing unit is of a predetermined type or types. When the power of the data processing unit is turned on, it is determined whether or not a memory unit has been attached to the data processing unit. Thereafter, data is read from a boot block of the memory unit. With reference to identification information of the boot block, it is determined whether or not the attached memory unit is a first type memory unit, such as a security type memory unit. If the attached memory unit is a security type memory unit, an authenticating process is performed. After the memory unit has been authenticated, contents data may be recorded or reproduced. If the attached memory unit is not a security type memory unit, a message that represents that the memory unit cannot be used may be issued. If the data processing unit is a non-security type data processing unit, a normal operation is performed regardless of whether the memory unit attached thereto is a security type or non-security type memory unit.