Secondary storage of a computer system is a medium used for storing data and files permanently.
Hard disk, diskette (floppy disk), CD (Compact Disk) and USB flash memory are examples of secondary storage.
Hard disks and diskettes are magnetic devices using particles capable of getting magnetized for saving data and reading it subsequently.
CD drives use optical beams for data creation and reading.
Hard disks, floppy disks, some CDs and SSD memories are rewritable.