WordNet (r) 3.0 (2006):

secondary storage
    n 1: a data storage device that is not the main memory of a
         computer [syn: auxiliary storage, external storage,
         secondary storage]

The Free On-line Dictionary of Computing (30 December 2018):

secondary storage

    Any non-volatile storage medium that is not
   directly accessible to the processor.  Memory directly
   accessible to the processor includes main memory, cache
   and the CPU registers.  Secondary storage includes hard
   drives, magnetic tape, CD-ROM, DVD drives, floppy
   disks, punch cards and paper tape.

   Secondary storage devices are usually accessed via some kind
   of controller.  This contains registers that can be directly
   accessed by the CPU like main memory ("memory mapped").
   Reading and writing these registers can cause the device to
   perform actions like reading a block of data off a disk or
   rewinding a tape.  See also DMA.

   Programs and data stored in secondary storage must first be
   loaded into main memory before the processor can use them.

   (1997-11-05)