Storage devices



1.4  Storage devices

Storage Devices

Storage devices hold data, even when the computer is turned off. The Physical material that actually holds data is called Storage medium. The surface of a floppy disk is storage medium. The hardware that writes data to or reads data from a storage medium is called a storage device. A floppy disk drive is a storage device.

The storage technologies are magnetic and optical.

Magnetic Storage Devices

How Magnetic Storage Works?

A magnetic disk's medium contains iron particles, which can be polarized—given a magnetic charge—in one of two directions. Each particles direction represents a 1 (on) or 0 (off), representing each bit of data that the CPU can recognize. A disk drive uses read/write heads containing electromagnets to create magnetic charges on the medium.


Before a magnetic disk can be used, it must be formatted—a process that maps the disk's surface and determines how data will be stored. During formatting, the drive creates circular tracks around the disk's surface, and then divides each track into sectors. The OS organizes sectors into groups, called clusters, and then tracks each file's location according to the clusters it occupies.

Disk Areas:

When a disk is formatted, the OS creates four areas on its surface:

1) Boot sector – stores the master boot record, a small program that runs when you first start (boot) the computer.

2) File allocation table (FAT) – a log that records each file's location and each sector's status

3) Root folders – enables the user to store data on the disk in a logical way

4) Data area – the portion of the disk that actually holds data.

The Various Devices that uses Magnetic technology are:


Diskette drives, also known as floppy disk drives, read and write to diskettes (called floppy disks or floppies). Diskettes are used to transfer files between computers, as a means for distributing software, and as a backup medium. Diskettes come in two sizes: 5.25-inch and 3.5-inch. Diskettes are available in different capacities, but the most common store 1.44 MB. High-capacity floppy disks offer capacities up to 250 MB

Hard Disks

Hard disks use multiple platters, stacked on a spindle. Each platter has two read/write heads, one for each side. Hard disks use higher-quality media and a faster rotational speed than diskettes. Removable hard disks combine high capacity with the convenience of diskettes. Hard disks store large amounts of data. New PCs feature hard disks with capacities of 10 GB and higher

Disk cartridges are like small removable hard disks, and can store up to 2 GB.

Magnetic tape systems offer very slow data access, but provide large capacities and low cost.

Optical Storage Devices

How Optical Storage Works?

An optical disk is high-capacity storage medium. An optical drive uses reflected light to read data. To store data, the disk's metal surface is covered with tiny dents (pits) and flat spots (lands), which cause light to be reflected differently. When an optical drive shines light into a pit, the light cannot be reflected back. This represents a bit value of 0 (off). A land reflects light back to its source, representing a bit value of 1 (on). 


In PCs, the most commonly used optical storage technology is called Compact Disk Read-Only Memory (CD-ROM). A standard CD-ROM disk can store up to 650 MB of data, or about 70 minutes of audio. Once data is written to a standard CD-ROM disk, the data cannot be altered or overwritten.

CD-ROM: Speeds and Uses Early CD-ROM drives were called single speed, and read data at a rate of 150 KBps. (Hard disks transfer data at rates of 5 – 15 MBps). CD-ROM drives now can transfer data at speeds of up to 7800 KBps. Data transfer speeds are getting faster. CD-ROM is typically used to store software programs. CDs can store audio and video data, as well as text and program instructions.


A variation of CD-ROM is called Digital Video Disk Read-Only Memory (DVD-ROM), and is being used in place of CD-ROM in many newer PCs. Standard DVD disks store up to 9.4 GB of data—enough to store an entire movie. Dual-layer DVD disks can store up to 17 GB. DVD disks can store so much data because both sides of the disk are used, along with sophisticated data compression technologies. Other Optical Storage Devices CD-Recordable (CD-R) drive lets to record own CDs, but data cannot be overwritten once it is recorded to the disk. Also known as WORM (Write once, Read many) CD-Rewritable (CD-RW) drive lets to record a CD, and then write new data over the already recorded data. 

PhotoCD technology is used to store digital photographs.

Flash drives/Pen drives: These are potable storage medium, with no movable part as in disk drives like hard disk of optical disk drives. They are fast to access and also referred as a static storage device.