Secondary storage means external storage. This storage device includes a magnetic disk, optical disk, magnetic tape and others. The most widely used external storage is as follows:

Logical Data Elements

If there is no systematic way to store and retrieve data, it is too difficult to get any information from an information system. Therefore, data resource should be organized in some logical manner. Data are logically organized into characters, fields, records, files, and database.

• Character: A character is the most elementary logical data element, whereas bit and byte are basic physical storage elements. A character consists of a single alphabetic letter, numeric digit, or special symbol. The character is equivalent to a byte.

• Field: The next higher level of data is the field. A field consists of a grouping of related characters. For example, a set of characters in a customer's name makes a name field. A field represents an attribute of an entity.

• Record: A record is a collection of related fields. The record represents a collection of attributes that describe an entity.

• File: A set of related records makes a file. For example, a customer file consists of many records of customers.

• Database: A database is an integrated collection of logically related files. A database combines several records previously stored in separated files.

Floppy disks are removable, direct access storage media inserted into disk drives. Floppy disks are flat, circular pieces of Mylar plastic that rotate within a jacket. These are also called flexible disks, floppies, Diskettes or simply disks.
Data and instructions are stored as forms of bits and bytes using the ASCII or EBCDIC data coding schemes. They are stored as electromagnetic charges on a disk surface. The two most common disks are 5 1/4-inch (5.25") and 3 1/2-inch (3.5") in their diameter. More efficient size, high storage capacity and sturdier design of a 3.5"-disk make microcomputers now use the 3.5" disks more.

Tracks/Sectors

• Track: The disk surface is divided into several concentric circles called tracks. The thinner the tracks, the more storage capacity of the disk. Data are recorded as tiny spots on these tracks. These tracks are closed concentric circles, not a single spiral. Each track has the same number of bits although the outer tracks are longer than the inner ones.

• Sector: The circular tracks are further divided into wedge-shaped sections known as sectors. The fields of data within a particular record are organized according to tracks and sectors on a disk. - graph

Hard / Soft Sectored Disks and Formatting

There are two kinds of sectors for a disk. A hard sectored disk is one and a soft sectored disk is the other. The hard sectored disk identifies sectors with some physical marks on the disk, whereas the soft sectored disk identifies sectors with sector identification contained in the recording tracks.
A format program places the sector identification on the disk. When a user buys a disk manufactured without tracks and sectors in place (this kind of disk does not have "Formatted" label on it), he or she must put the tracks and sectors using a format program.

Capacity

• DS/DD (2S/2D) Disk: DS/DD means Double Sided Double Density. This is a floppy disk recorded on both of its sides (DS) and has twice the capacity of the prior format (DD). For the IBM and IBM compatible PCs, DD/DS disk usually has 40 tracks on each side and each side contains 9 sectors of 512 bytes each.
  DS/DD: 360KB
  DS/DD: 720 KB (PCs), 800 KB for Mac formats.

• DS/HD Disk: DS/HD means Double Sided High Density. An HD disk has increased storage capacity usually with more tracks per square inch. For IBM and IBM compatible PCs, an HD disk is usually formatted to have 80 tracks on each side. Each side then contains 18 sectors of 512 bytes each - double capacity of a DD disk.
  DS/HD: 1.2MB
  DS/HD: 1.44 MB (PCs), 1.44 MB for Mac formats.

Floppy Drives

A floppy drive grabs a disk at its center and spins it inside its plastic jacket. The floppy drive obtains stored data and instructions from a floppy disk and stores them onto the disk. The drive is made up of a box with a slot into which a user inserts a disk. The slot has a drive gate. This drive rotates the disk with a motor inside the drive. Electronic read/write heads "read" data from the disk and "write" data to it while the disk rotates.
A microcomputer usually has internal floppy drives inside the computer cabinet, but it sometimes has external floppy drive, a separate component outside the cabinet.

Working Process of Floppy Drive

• Inserting disk: Inserting a disk to let it work.
  
• Closing gate: After inserting, the drive gate should be closed. This places the disk around a spindle and holds it.
  
• Revolving: Spinning the disk inside the jacket.
  
• Reading/ Writing: Read- write heads on an access arm transfer data signals from disk to computer or computer to disk.
  
• Seek: Seek operation is to let the access arm move the read-write head to the requested track on the disk.
  
• Search: Search operation is to let the drive rotate the disk to the proper position.

Hard Disks (Hard Drives)

A hard disk is a magnetic disk made of metal and covered with a magnetic recording surface. Hard disks come in removable and fixed varieties that hold from several hundreds of megabytes to several gigabytes. They are tightly sealed to prevent any foreign matter from getting inside which causes head crash.

Interface Type

• IDE(Integrated Drive Electronics): An IDE interface has a disk drive that contains its own controller electronics. The IDE interface is also called an AT and XT interface. IDE-ready motherboards have a 40-pin socket that connects directly to an IDE drive eliminating the use of an expansion slot.

• SCSI (Small Computer System Interface): SCSI is pronounced scuzzy. This is an 8-bit-bus peripheral interface for up to seven peripherals. The SCSI bus allows any two devices to communicate at one time (host to peripheral, peripheral to peripheral). SCSI provides high-speed (4MB/sec.), parallel data transfer and multiple peripheral connections while taking only one expansion slot.

Access Time

• Access time: This is an average time taken to complete the transfer of data after the request instruction has been enacted. Today's fast hard drives have access times under 10 milliseconds (ms). Access time is made up of the following four times.

• Seek Time: This is the time taken to move an access arm to a certain track on a disk after the computer requests data. Most of the access time is made up of seek time.

• Head Switching Time: The time taken for changing from one read/write head to another to read from or write on another part on a disk.

• Search Time: It is also called rotational delay time. This is a time required for the read/write head to locate particular position on a track.

• Data Transfer Time: This is the time for data to be transferred from the disk to primary storage or vice versa.
  

Different Forms of Hard Disks

• Internal Hard Disk: Internal hard disk is made up of several metallic platters, a motor, an access arm and read-write heads sealed inside a container.
  An internal hard disk is looked like a part of a system unit inside a computer cabinet. There are two sizes of drives (5.25" and 3.5" in a diameter). 3.5" hard disks are faster because the access arm travels shorter distances across the diameter of the disk.
  Internal hard disks have advantages over flexible disks. They are high capacity and speed. The disadvantage of internal hard disks or hardcards is that they have only a fixed amount of storage and cannot be easily removed.

• External Hard-Disk Drives: This is a drive that is not built into the system cabinet of microcomputers. External hard disk drives are treated as peripherals. Using external hard disk drives, we can expand the hard disk capacity when all available drive bays are occupied.

• Hard Disk Cartridges: A cartridge is a removable storage module, so a hard disk cartridge contains disks in the module. They can be removed from a dock easily and can give fast access to large data. An internal or external dock is available.
  In internal hard disks and external hard disk drives, the storage capacity is fixed, but in the hard disk cartridge, the capacity limitation of storage does not exist. That is, a user may add more cartridges any time.

• Removable Drives with Cartridges: Today's advanced technology allows a new form of removable storage. The technology combines the function of the hard disk drive and the convenience of the hard disk cartridge. This form consists of a removable drive and several cartridges.
  The removable drive looks like an external floppy disk drive. And the cartridge resembles a floppy disk and allows users to add 100 MB or 1 GB at a time. It allows an SCSI connection as well as a parallel port connection. Its low cost is a big advantage.

More advanced technology created a new disk storage forms. Optical disk is a disk written and read by laser beam. This optical disk has a great impact on today's storage technology. Optical disk does not spin, does not need to move access arms and read/write heads, because a laser beam can be moved electronically. The capacity of the storage is considerably greater than their magnetic disk counterparts, and optical disk storage may eventually replace all magnetic tape and disk storage.
Then how do they work? To write data, a laser beam burns tiny cavities into the surface of a disk to mark bits for data. To read the data, a laser beam scans these areas. There are three forms of optical disks available:

• CD-ROM: CD-ROM (compact disk read only memory) is an optical disk storage that contains text, graphics and hi-fi stereo sound. CD-ROM is a 4.75-inch optical disk storage that can store around 650 MB of data. CD-ROM disk is almost the same as the music CD, but uses different forms of track for data. A CD- ROM drive can read music CD, but a CD player cannot read CD-ROM. CD-ROM is a read-only disk that cannot be written on or erased by the user. In CD- ROM standard, data (text or pictures) cannot be viewed with audio play simultaneously. CD-ROM XA standard can do.

• WORM: A WORM (write once, read many) disk is an optical disk that written on just once by the user's environment and then cannot be overwritten. A WORM disk is ideal for use as archive because it can be read many times, but the data cannot be erased. The storage capacity of WORM disk ranges from 400 MB to 6.4 GB.

• Erasable Optical Disks: This is an optical disk that can be erased and written on repeatedly. An erasable optical disk has a great deal of data capacity. It can store up to 4.6 GB. An erasable optical disk functions like a magnetic disk and has huge capacity, so it will replace the magnetic disk in the future.

A magnetic tape is a tape coated with a magnetic material on which data can be stored. This is a sequential storage device that is usually used for a backup purpose. A magnetic tape is slower than direct access storage such as disk, because it is sequential access storage. The biggest advantage of magnetic tape is the cost. It is much less expensive than magnetic disk and optical disk. An advanced tape backup technology (DAT) is available these days.

A cache is a specially designed buffer storage used to improve computer performance by reducing access time. It holds instructions and data that are likely to be needed for next operation by the processor. The cache copies frequently accessed data and instructions from primary storage (main memory) or secondary storage (disks).

• Disk Cache: A disk cache is in a reserved segment of primary memory or in an extra memory on the disk controller card. It contains a large block of frequently accessed data copied from a disk. The data in a disk cache can be used to fulfill the following data requests from a processor in a high speed. The disk cache lets the processor avoid a slow disk access.

• Memory Cache: A memory cache is a high-speed memory storage between memory and the CPU. It is smaller and much faster than main memory (primary storage). The memory cache copies blocks of instructions and data from the main memory so that execution and data updating are performed in the higher-speed memory bank.

File

In word processing, a file is a single document. In computer graphics, a file is a set of image descriptors for one picture. In a database, a file is a collection of records.

Read-Write Head

A device that reads and writes data on a magnetic disk. For writing, the surface of the disk is moved past the read/write head. Bits of data are recorded as tiny, magnetized spots of positive or negative polarity, by discharging electrical impulses at the appropriate times. For reading, the bits on a disk induce an electrical current across the read head.

Access arm

This is a mechanical arm that moves the read/write head across the surface of a disk. The access arm is directed by the operating system to move the read/write head to a specific track on the disk.

Fast SCSI

Fast SCSI provides 10MB data transfer through an 8-bit-bus.

Wide SCSI

Wide SCSI provides up to 40MB transfer speed with a secondary "B" cable for 16-bit and 32-bit data.

Tightly Sealed Container

A hard disk is enclosed in a tightly sealed container. The reason is to prevent any foreign material from getting inside. The read-write head rides on a cushion of air about 0.000001- inch thick. It is so thin that a smoke particle, fingerprint, or human hair could cause a head crash.

Head Crash

This is a physical destruction of a hard disk. Misalignment or contamination with dust can cause the read/write head to collide with the disk's recording surface. The head crash destroys all or part of data on the disk.

Hardcard

A hardcard is a circuit board with a disk that plugs into an expansion slot in the computer. The card is only 1-inch-thick. This can be an alternative to the external hard-disk drives.

CD-ROM XA

CD-ROM eXtended Architecture is an extension of the CD-ROM standard that allows for audio to be played concurrently while viewing data. CD-ROM XA uses a standard CD-ROM player, but requires a CD-ROM XA controller card in the computer.

DAT

Digital Audio Tape is a technology to record data using digital recording technology on a small magnetic tape. A 4mm-DAT drive holds 2 GB or more.