Primary memory has a limited storage capacity and it is not permanent that is why to store large volume of data permanently secondary memory or storage devices are used. They are of many types and they vary in the capacity of storage, speed of data access and media of storage. Nowadays magnetic disks and optical disks are commonly used. 

1. Magnetic Disk: 

Magnetic disk allows the storage and retrieval for contents of the disk from anywhere at a moderate speed. Magnetic Disks available in various size and storage capacity but the storage media and data access mechanism are similar. 

The storage media is circular platters or disks coated with magnetic material. It consists of a spindle capable of rotating with the help of an electrical motor and a read/write head. 

(a) Floppy disk: Floppy means flexible or soft, it uses flexible disk. Its storage capacity is 1,44MB and slower in data transfer rate. 

(b) Hard Disk: Instead of flexible or soft disk it uses rigid material hence the name hard disk. Its storage capacity and data transfer rate are high and low access time. These are more lasting and less error prone. The accessing mechanism and storage media are combined together in a single unit and connect to the mother board via cable. 

Therefore we call it as hard disk drive (HDD). It contains one or more rigid platters coated both sides with a special magnetic material and a spindle. Datas are recorded on either surface of the disk except the outer side of last and first disk. Each surface will have one or more read/write heads (fixed head or movable head). 

The spindle is attached to a motor that rotates at high speed typically 7200 rotation per minute (rpm). A floppy disk rotates only at 300 rpm. 

2. Optical Disk: The high power laser uses a concentrated, narrow beam of light, which is focuses and directed with lenses, prisms and mirrors for recording data. 

The optical disks are given below: 

(a) Compact Disk Read Only Memory (CDROM): The data in CDROM is imprinted.by the manufacturers. The user cannot erase or write on the disk but user can only read its contents. CDROM is written in a single continuous spiral unlike magnetic disks that uses concentric circles. Its storage capacity is 700MB. 

(b) Erasable Optical Disk: The disadvantage of CDROM is that we cannot change or erase the contents. But erasable disks can be changed and erased. 

(c) Digital Versatile Disk: It is capable of storing up to 4.7GB and more faster. 

(d) Blu-ray Disc: It is used to read and write High Definition video data as well as to store very huge amount of data. While Cd and DVD uses red laser to read and write but it uses Blue Violet laser, hence the name Blu ray disc. The blue-violet laser has shorter wavelength than a red laser so it can pack more data tightly.

3. Semiconductor storage (Flash memory): It uses EEPROM chips. It is faster and long lasting. 

1. USB flash drive: It is also called thumb drive or pen drive. Its capacity varies from 2 GB to 32 GB. 

2. Flash memory cards: It is used in Camera, Mobile phones, tablets etc to store all types of data.