Secondary Storage Device

 Secondary memory, also known as secondary storage, is the slower and cheaper form of memory. CPU does not access the secondary memory directly. The content in it must first be copied into the primary storage RAM for CPU to process. Secondary memory devices include hard drives, floppy disks, CDs and CD-ROMs etc. Secondary memory is the place where programs and data are kept on a long-term basis.

Differentiate between primary and secondary memory

 

Primary memory		Secondary memory
1.	It connects directly to the processor.	1.	Not connect directly to the
		processor.
2.	They are expensive.	2.	They are cheaper.
3.	They are having low capacity.	3.	They are having high capacity.
4.	They are fast.	4.	They are slower.
5.	They are semiconductor memory.	5.	They are optical or magnetic
		memory.
6.	Example:- RAM, ROM etc.	6. Example:- Hard disk, floppy disk,
		CD etc.

1.      Magnetic Disk

Magnetic disk is the most widely used storage medium for computer. A magnetic disk offers high storage capacity, reliability and fast to access data. It access data randomly. Magnetic disk is flat circular plates of metal or plastic, coated on both side with iron oxide. The types of magnetic disk are hard disk, floppy disk, pen drive etc.

2.      Hard Disk

The hard disk drive is the most popularly used secondary memory device. The main reason for this is the access speed and the reliability of the data it can offer. In the case of the tape drives the access speed is much low and the data transfer is comparatively low than the hard disk drive.

The information that is required to boot a computer is stored in the hard disk boot sector. Also the importance of the hard disk drive is to store the backup of the data or any information that is created by the user. The hard disk specification should also match the expectations of the computers; that is the storage capacity and the access speed.

The internal organization of the any hard disk drive consists of four parts primarily. They are PLATTERS, HEAD ARM, CHASIS, and HEAD ACTUATOR. The hard disk drives are also available in two different types that are internal and also the external hard disk drives. The internal hard disk drives are used for the storage of the data in the computer case. They are not portable and usually are inside the case. The external hard disk drives are portable can be connected to other computer systems as well. There is a hard casing over the hard disk.

The platters are rotated at very high speed varying from 3,000 rpm (revolution per minute) to 15,000 rpm. Information is written to, and read from a platter stack, called read –write head. The read –write head is used to detect and modify the magnetization of the material immediately under it.

The hard disk is a non-volatile and random memory of a computer system. The hard disk has very high storage capacity. A hard disk has a storage capacity in terms of Giga bytes such as 80 GB, 320 GB, 500 GB etc.

ADVANTAGES OF HARD DISK

·         It can store huge amount of data in terms of MB to GB.

·         It is protected from dust, fires etc.

·         It stores data safely in comparison to other devices.

·         External hard disks are portable.

·         Data access time is less in comparison to floppy disk , magnetic tape etc

 

3.      FLOPPY DISK

The floppy disk is also called a diskette. Floppy disk is used to transfer data from one computer to another, increase hard disk space by saving information to floppy, backup storage media. The disk is cheap, easy to handle and less storage capacity. Floppy disks have largely been replaced with CD, DVD. Advantages of Floppy disk

·        Direct information encoding onto disk.

·        No handling of bulky media.

·        Error correction is easier than in cards and paper tapes.

·        It can be re-used many times.

4.      Magnetic Tape

Magnetic Tape is a sequential storage device of computer. It is not common storage device now a day. It is mainly used in mainframe computers and super computers. It is slow and sequential flow of data. It is very useful for storing huge volume of data economically. A magnetic tape is a continuous trip of plastic. It is coated with magnetic oxide which can be magnetized to record information as a series of magnetized / non-magnetized spots to represents binary 0 and 1. Extracting information from a file is known as reading.

The magnetic tape used for computer is made from the same material used for audio tape and video tape. Today, it is used mainly to provide backup or duplicate storage. At present, it is available in cassette forms, which is called cartridge tape. The storing capacity can be 500 MB, 2 GB, 4 GB or higher. Data density is 120 Kbits per inch. Speed is 200 inch per second and data transfer rate is 240 KB per seconds.

 

5.      MEMORY CARD (FLASH MEMORY)

A memory card is a small storage medium used to store data such as text, pictures, audio and video, for use on small, portable or remote computing devices. It is non-volatile memory which means that data is stable on the memory card by a loss of power source, and does not need to be periodically refreshed. Because memory cards are solid state media, they have no moving parts, and therefore, are unlikely to suffer mechanical difficulties.

Memory cards offer a number of advantages over the hard disk drives. They are much smaller and lighter, extremely portable, completely silent, allow more immediate access, and are less prove to mechanical damage. The storage capacity is from MB to GB. They are commonly used in many electronic devices, including digital camera, mobile phones, Laptops, MP3 Players, and Video Game Consoles.

 

6.      OPTICAL DISK

An optical disk is an electronic data storage medium that can be written to and read using a low powered laser beam. It is mainly used for music, movies and software programs. It is compact, light weight and durable. Optical disks are CD, DVD etc.

·         CD-ROM (Compact Disk-Read Only Memory)

CD-ROMs are popularly used to distribute computer software including games and multimedia applications through any data can be stored. The diameter of CD-ROM is 12 cm, thickness is 1 mm and weight is 18 gm. The storage capacity of CD-ROM is 650 MB. It is only used for read only purposed in computer system.

·         CD-RW (Compact Disk Re-Writable)

A CD-RW is a re-writable optical disk. CD-RW Disk required more sensitive laser optics. CD-RWs are composed of a poly carbonate plastic, a thin reflecting metal coating, and a protective outer coating. In a CD-RW the dye is replaced with an alloy that can change back and forth from a crystalline form when exposed to a particular light, through a technology called optical phase change. The pattern created is less distinct than those of others CD formats, requiring more sensitive device for playback.

·         VCD (Video Compact Disk)

VCD is a compact disk format based on CD-ROM that is specifically designed to fold MPEG-1 videos data and to include interactive capabilities. Each VCD disk holds 72 – 74 minutes of video and has a data transfer rate of 1.44 Mbps. VCD‟s can be played on a VCD player connected to a Television set or computers, on a CD player, on same CD-ROM drives, and same DVD player.

·         DVD (Digital Video Disk)

DVD is an optical disk storage media format. DVD disks offer higher storage capacity. DVD disks are used in DVD video consumer digital video formats and in DVD audio consumer digital audio formats. DVD disks containing other types of information may be referred to as DVD data disks. It can store any types of data such as text, graphics, audio, video etc.

 

7.  External storage devices

An external storage device, also referred to as auxiliary storage and secondary storage, is a device that contains all the addressable data storage that is not inside a computer's main storage or memory. An external storage device can be removable or non-removable, temporary or permanent, and accessible over a wired or wireless network.

External storage enables users to store data separately from a computer's main or primary storage and memory at a relatively low cost. It increases storage capacity without having to open up a system.

·         External Hard Disk

It is a hard disk that is connected outside from a computer. External hard disks usually are used in addition to internal hard drives in order to store more data. They are portable i.e. carry from one computer to another computer. The external hard drive is connected to the computer via an interface cable, which allows the external hard drive to communicate with the computer so that data can be passed back and forth.

·         Zip Disk and Jaz Disk

A zip disk and Jaz disk are a small, portable disk drive used primarily for backing up and archiving personal computer files. They are used to move data from one computer to another computer.