SECONDARY STORAGE DEVICES

 Primary storage devices of today's computer have the following limitations:

• Limited capacity. It is often necessary to store many millions, sometimes billions, and even trillions, of bytes of data in a computer. Unfortunately, the storage capacity of today's computers is not sufficient to store large volumes of data.
• Volatile. Primary storage is volatile and it loses data stored in it in case of power off or interruption of power. However, computer systems need to store data on a permanent basis for several days, several months, or even several years.

To overcome this limitation of primary storage, almost all computers use additional memory called auxiliary memory or secondary storage. Secondary storage is non-volatile and has a low cost per bit stored, but operates at speeds far slower than that of primary storage.

Secondary storage devices are classified as:

1. Sequential access devices. A sequential access device is one in which arrival at the desired location is preceded by sequencing through other locations so that the access time varies according to location. Magnetic tape is an example of the sequential access device.
2. Direct access devices. A direct access device is one in which we can reach and access any storage location at random, and approximately equal access time is required for accessing each location. Magnetic disks, optical disks, and memory storage devices are examples of direct access storage devices.

MAGNETIC TAPES

Magnetic tape is the most popular storage medium for large data sets accessed and processed sequentially. Computer systems also used it as a backup storage for data storedon on-line storage devices such as a hard disk.

Magnetic tape medium is a plastic ribbon usually 1/2inch or 1/4inch wide and 50 to 2400 feet long. Its surface has a coating (such as iron oxide or chromium diaoxide) that can record data by magnetization. Data is recorded on the coated surface as tiny, invisible magnetized and non-magnetized spots (representing 1s and 0s). Tape ribbon itself is stored in reels, or a small cartridge, or cassette.

MAGNETIC DISKS

Magnetic disks are the most popular direct access secondary devices.

A magnetic disk is a thin, circular plate of metal or plastic. Its surfaces on both sides have a coating (such as iron oxide) that can record data by magnetization. Data is recorded on its coated surfaces as tiny, invisible magnetized and non-magnetized spots (representing 1s and 0s). It uses a standard binary code, usually 8-bit EBCDIC, for recording data.The disk itself is stored in a specially desgined protective envelope, or cartridge, or several of them are stacked together in a sealed, contamination-free container.

Magnetic disks are broadly classified as:

• Floppy disks
• Hard disks

OPTICAL DISKS

An optical disk storage system consists of metalic or plastic disk coated with highly reflective material. It uses laser beam technology for writing/reading of data to/from disk surface. Optical disks are also known as laser disks or optical laser disks because they use laser beam technology for data read/write. Modern computer systems use optical disks as random access medium for high capacity secondary storage because of their capability to store very large amount of data in a limited space.

Commonly used optical disks are CD-ROM, WORM (CD-R), CD-RW, and DVD.

MEMORY STORAGE DEVICES

With the advent of efficient manufacturing technologies, price of memory chips has fallen considerably. As a result, storage manufacturers have introduced a new breed of electronic secondary storage devices (which do not use any mechanical component for operation). They often provide applications (like music player, etc) on top of these storage devices for better marketability. Some of the popular ones are described below.

Flash Drive (Pen Drive)

Flash drive  is a compact device of the size of a pen. It comes in various shapes and stylish designs and may have different added features (such as with a camera, with a built-in MP3/WMA/FM Radio playback for music on the go, etc.). It enables easy transport of data from one computer to another.

A user simply plugs it into a USB(Universal Serial Bus) port of a computer. The computer it automatically as a removable drive. Now the user can read, write, copy, delete, and move data from the computer's hard disk to the flash drive or from the flash drive to the hard disk drive.

Memory Card(SD/MMC)

Similar to flash drive, flash memory based cards are available as removable storage device in different types of electronic equipment. Some of the most popular ones as Secure Digital (SD), Multimedia Card (MMC). Storage capacity of these cards range from 8MB to 128GB.