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COURSE NAME: CXC Information Technology TOPIC: Secondary Storage LECTURE 3 SECONDARY STORAGE Secondary storage is that type of storage that is mainly concerned with holding data permanently (for later use). Secondary storage is also known as: Auxiliary Storage Backing Storage Secondary Storage is referred to as “non-volatile” storage. When the power goes the information stored on the secondary storage device remains. Types of Secondary Storage Medium Magnetic Disk There are two main types of magnetic disk: Floppy Disk Hard Disk Floppy Disk – a portable type of storage. The most popular size 3 ½” is used to store up to 1.44 megabyte of data. Floppies consist of a thin flexible plastic disk that is coated with a magnetisable substance like oxides of iron or chromium (or a combination of the two). Floppy Disk Hard Disk – These are otherwise called “Fixed disk”. It stores all the information that is on your computer. The hard disk has a hard platter that holds the magnetic medium, as opposed to the flexible plastic film found in tapes and floppies. Prepared by Miss N. Nembhard 1 Hard Disk Although both floppies and hard disk use random access or direct access (this moves to any point on the disk surface almost instantly), the platter in the hard disk rotates approximately twenty times faster than that of a floppy disk. As a result the hard disk is considered on of the fastest secondary storage. Magnetic Tape – This is similar to the continuous reel of recording surface in an audio videocassette. It is made from magnetisable plastic. You cannot go directly to an item of data on a tape as you can on a disk. It is necessary to start at the beginning of the tape and search for the data as the tape goes past the read head. This kind of access is known as “Serial access” or “Sequential Access”. Magnetic Tape Zip Disk – This is a 3.5" removable disk drive from Iomega. Zip disks come in 100MB, 250MB and 750MB varieties, with the latter introduced in 2002 using USB and FireWire interfaces. Zip disks are a floppy-like technology that uses design concepts from hard disks. Zip Disk Prepared by Miss N. Nembhard 2 A sector is a pie-shaped division of each disk track. It is the smallest unit that can be accessed on a disk. When a disk undergoes a low-level format, it is divided into tracks and sectors. The tracks are concentric circles around the disk and the sectors are segments within each circle. For example, a formatted disk might have 40 tracks, with each track divided into 10 sectors. The operating system and disk drive keep tabs on where information is stored on the disk by noting its track and sector number. Modern hard disk drives use a technique called zoned-bit recording in which tracks on the outside of the disk contain more sectors than those on the inside. A sector that cannot be used due to a physical flaw on the disk is called a bad sector. Track - The path on a tape, disk or drum on which data are stored. On a disk these paths are concentric circles; on a tape there are several tracks parallel to the edge of the tape; on a drum there are bands of equal size along the circumference of the drum. A track is a ring on a disk where data can be written. A typical floppy disk has 80 (double-density) or 160 (high-density) tracks. For hard disks, each platter is divided into tracks, and a single track location that cuts through all platters (and both sides of each platter) is called a cylinder. Hard disks have many thousands of cylinders. Each track is further divided into a number of sectors. The operating system and disk drive remember where information is stored by noting its track and sector numbers. Flash Drives – These are small, portable flash memory card that plugs into a computer’s USB port and functions as a portable hard drive. USB flash drives have less storage Prepared by Miss N. Nembhard 3 capacity than an external hard drive, but they are smaller and more durable because they do not contain any internal moving parts. USB flash drives also are called thumb drives, jump drives, pen drives, key drives, tokens, or simply USB drives. A memory card (sometimes called a flash memory card or a storage 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. Most available cards have constantly powered nonvolatile memory, which means that data is stable on the card, is not threatened 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. Optical Disk Storage This technology uses laser light to read or write (burn) data onto an optical medium. Optical disks use random access. Different Varieties of CDs exists such as CD-R, CD RW, CD ROM etc and can store over 700 MB of data. Compact Disk Read Only Memory (CD-ROM) – The CD drive reads data with a beam of light so narrow that the information can be squeeze together very tightly. As a result, the CD storage capacity is large. Prepared by Miss N. Nembhard 4 CD Compact Disk Recordable (CD-R) – Allows you to write onto specially manufactured disk. Once the information is stored on this disk it cannot be changed. This CD can store up to 80 minutes or 700 MB of data. Compact Disk Rewritable (CD-RW) – Data can be read and written to the disk. Information written can be erased many times. Digital Versatile Disc (DVD) – It is faster and can hold more data than CDs. It can store audio, video and data. DVD Magneto Optical While magnetic tape, floppy disk, hard disk uses electro-magnetism technology and the newer storage devices uses that of light or the optical technology. Magneto Optical uses both elector-magnetism and optical technology. This device is considered a hybrid. Magneto Optical The most common secondary storage devices for use with Microcomputers are the floppy disk and the hard disk. Recording Techniques Used by Different Secondary Storage Devices Storage Device Magnetic tape, Magnetic disk CD, DVD Magneto Optical Recording Technique Magnetism Laser Optical Magnetism and Laser Optical Prepared by Miss N. Nembhard 5 Activity 1. Indicate the type of access method used by the following devices: Device Magnetic Tape Floppy Disk Hard Disk Compact Disk DVD Magneto Optical Access Method 2. Create a table showing different types of magnetic and optical secondary storage devices. Prepared by Miss N. Nembhard 6