An example of the invention classifies disks drives based on their purpose and associating power-saving policies in multiple classes. The system implements Power On Demand, where a reduced power mode is enabled for every individual component of a storage subsystem. In addition, an embodiment of this invention extends a few power modes used in the prior art and allows almost infinite number of power modes when instructing a disk drive (via its I/O interface such as a SCSI or fiber channel interface) to enter a certain power saving mode. Furthermore, an embodiment of invention teaches a system and methods to save power in a disk system comprising a plurality of disk controllers and a plurality of disk drives, arranged in a plurality of arrays, where each array includes several (e.g., 4-16) disk drives.