A storage medium stores a set of program instructions including: acquiring a number of at least one virtual unit with which at least one of a plurality of storage units is associated, the number of at least one virtual unit being set smaller than a total number of the plurality of storage units; receiving, from an operating system, a first command that specifies one of the at least one virtual unit; registering, as an accessible storage unit, a single storage unit from among the at least one of the plurality of storage units that is associated with the one of the at least one virtual unit specified by the first command, the single storage unit being in condition for communicating data with a data processing device; and outputting, to a peripheral device, a second command that specifies the accessible storage unit based on the first command.