Competencies

Discuss the different types of storage media: floppy disks, hard disks, optical disk, and magnetic tape.

Describe today's standard floppy disk and compare it to higher- capacity floppy disks.

Describe the following kinds of hard disks: internal hard disks, hard-disk cartridges, and hard-disk packs.

Describe ways to improve hard-disk performance: disk caching, redundant arrays of inexpensive disks, and file compression and decompression.

Describe the different types of optical disks.

Describe other kinds of secondary storage: magnetic tape, Internet drives, and solid state storage.

Introduction

• Secondary storage devices are used to save, to back up, and even to transport files consisting of data or programs from one location or computer to another.
• The need for storage continues to grow due to higher demands of users to store more digital media such as videos, music, and images.
• Data is stored on secondary storage in digital or machine code, so it doesn’t need to be translated from the 1s & 0s when it is sent to the CPU for processing.

Storage

• RAM (Random Access Memory) is called primary storage since it is used directly by the CPU for processing data and program instructions.
• RAM is volatile or temporary storage (once the power is turned off, the contents are lost).
• Secondary storage provides permanent or non-volatile storage.
• Secondary storage devices read and write the data onto the storage medium.
• Reading is the process of retrieving/accessing information from the secondary storage device.
• Writing is the process of storing/saving the information to the secondary storage device.

 

Important characteristics of secondary storage include:

• Media or medium: the actual physical material that holds the data
• Capacity: how much the particular storage medium can store, typically measured in MB, GB, and TB
• Access time: the amount of time to read and/or write data and programs to the storage medium

Floppy Disks

• Floppy disks (aka floppies, diskettes, disks or flexible disks) are portable, flexible and removable storage media.
• They use flat circular pieces of Mylar plastic coated with a magnetic material that rotate within a jacket.
• Data is stored as electromagnetic charges by the presence or absence of these charges, using the ASCII or EBCDIC binary codes.

Traditional Floppy Disk

• Traditional disk is the 1.44 MB 3 ½” disk, labeled as 2HD meaning "two-sided, high-density".
• They have a thin exterior jacket made of hard plastic to protect the flexible disk inside.
• These disks have a capacity of 1.44 megabytes – equal to 400 typed pages.
• Density refers to how tightly the bits can be packed on the medium.
• A Shutter slides to provide access to the plastic medium that comes under the read/write head of disk drive.
• Labels can be applied to the external surface of the disk to identify the contents
• A Write-Protection Notch can be moved to protect the disk from accidentally writing over it.
• Data is recorded on a disk in rings called tracks – closed concentric circles.
• Each track is divided into invisible wedge-shaped sections known as sectors.
• Disks that don't have tracks and sectors must be adapted to the type of microcomputer by a process called formatting or initializing.

High Capacity Floppy Disks

• High Capacity Floppy Disks (aka floppy-disk cartridges) have capacities of much higher than traditional floppies.  Three leading types include:
• Zip Disks (sold by Iomega) have 100-, 250-, or 750 MB capacities and external zip drives connect to the PC via parallel or USB ports.  The disks are slightly thicker than traditional floppies, so they require special disk drives. Zip disks are widely used to store multimedia, database, large spreadsheet and text files.
• SuperDisks (from Imation) have a 120 MB or 240 MB capacity, and the drives can also read and store data on traditional 1.44 MB floppies. Popular for use with notebook computers.
• HiFD disks (from Sony Corporation) have 200 MB capacities.  The main advantage is the drives can also read traditional 1.44 MB floppies.

Hard Disks

• Hard disks use a thicker, rigid metallic platter for the base medium..
• Hard disks store and retrieve information much faster and have a greater capacity.
• Read/Write heads are very sensitive and ride a 0.000001 (one 1 millionth) inch cushion of air above the disk.
• A “head crash” occurs if the R/W head makes contact with the surface or particles on the surface (human hair, dust, fingerprint) of the disk, and it’s a disaster.

Internal Hard Disk

• Also known as a fixed disk
• Located inside the system unit and used to store the operating system and major applications like Word and Excel.
• Consists of one or more metallic platters sealed inside a container.
• The container contains a motor for rotating the disks, an access arm and read-write heads for reading and writing data.
• Typically mapped as the “C:” drive
• Advantages are speed and capacity: a 100 GB HD can hold as much as 70,000 traditional 1.44 MB floppies and are also much faster than floppy disks.
• Access speeds are measured in milliseconds (ms) e.g. 10 ms
• Disk rotation speeds are measured in RPM (rotations per minute) e.g. 5,400 RPM
• You should perform routine maintenance on your hard drive using programs such as Microsoft’s Disk utility programs.

Hard-Disk Cartridges

• Hard disks have fixed amount of storage and they cannot be easily removed.
• Hard disk cartridges are easy to remove and are known as removable hard disks and are useful to protect sensitive information.
• Storage amount is limited only by the number of cartridges you use
• Cartridges typically hold 2-20 GB of storage.
• Two well-known hard-disk cartridges are Jaz and Peerless disks from Iomega.
• PC Card Hard disks are credit card sized hard-disk cartridges with capacities up to 10 GB.
• Examples include IBM’s Microdrive and Hitachi's PC Card hard drive.

Hard-Disk Packs

• Hard disk packs are removable storage devices used to store massive amounts of information without duplicating the drive mechanism.
• They may have up to 11 large disks with 20 recording surfaces.
• Typical use is in large mainframe shops like banks and insurance companies.

Performance Enhancements

• Disk Caching: frequently used data is read from hard disk into memory cache. When needed, data is read directly from memory. Transfer rate from memory is much faster than hard disk which improves the transfer rate to the CPU by up to 30%.
• Redundant Array of Inexpensive Disks (RAID): improves performance by expanding external storage. Groups of inexpensive hard-disk drives are grouped together using networks and special software and is treated as single large-capacity hard disk. While it costs more to have a RAID system, it improves storage reliability.  RAID systems are typically used for network servers.
• File Compression and File Decompression: increase the amount of storage available on the disks by reducing the amount of space required to store data and programs. File compression programs scans the files for repeating patterns and replace them with a token. Popular programs for compressing files include WinZip and PKZip.  The smaller size comes at a price, since it takes a little longer to uncompress the data.

Optical Disks

• Optical disks can hold close to 17 GB of data – enough to store over several million typewritten pages or a medium sized library on a single disk.
• Optical disks use reflected light rather than magnetized spots.
• Binary 1s are represented by flat areas called “lands” and 0s are represented by bumpy areas called “pits” on the disk surface.
• The disk is read by a laser that projects a tiny beam of light on these areas. The amount of reflected light determines whether the area represents a 1 or 0.
• Unlike hard disks that have concentric tracks, optical disks have a single spiral track that is divided into equally sized sectors for storing data.
• The most common sized optical disk is 4 ½ inches, and data is stored on these disks in different formats. The two most common are:

Compact Disc (CD)

• One of the most widely used optical formats
• Typically store 650 MB to 1 GB (1,000 MB) on one side of a CD
• Rotational speed determines how fast data can be transferred to the CPU
• 24X (24 speed) CD can transfer data at 3.6 MB per second
• 32X (32 speed) CD can transfer data at 4.8 MB per second

CD-ROM (Compact Disc – Read Only Memory)

• is similar to a commercial music CD
• RO means it can not be written over by the user
• Typically used to deliver large databases, references, or software applications

CD-R (Compact Disc – Recordable)

• write once, read many times but cannot be written on or erased
• CD-R drives also known as CD burners typically use these to archive data or record music

CD-RW (Compact Disc – Rewritable)

• write many times, read many times
• Used to create and edit multimedia presentations
• Typically cost a little more than CD-R

Digital Versatile/Video Disc (DVD)

• A newer format similar to CDs except that more data can be packed in same space and is replacing CD optical disks
• DVDs can store 4.7 GB to 17 GB on a single disk (17 times the capacity of a single CD)

DVD-ROM (Digital Versatile Disc - Read Only Memory)

• Written at manufacturing plant, read many
• Provide over 2 hrs of high-quality video and sound and typically used for video distribution

DVD-R (DVD Recordable)

• Write once, read many
• Tend to cost more than CD writable disks
• Used for archiving data and writing video files

DVD+RW (DVD Rewriteable) DVD-RAM

• Write many, read many
• Still working on setting a standard format

 

Other Types of Secondary Storage

Magnetic Tape

• The two different approaches of external storage are: sequential access and direct access.
• Tapes only provide slower sequential access, where disk systems provide fast direct access.
• With tapes information is stored in sequence, such as alphabetically.
• Magnetic tape is typically ½-inch wide and ½-mile long.
• Advantage with tape is virtually unlimited storage (just add another tape), it’s reliable, and it’s inexpensive per MB stored.
• Disadvantage is it’s somewhat slow, and limited to sequential access
• Often used to back up disk storage, especially for networked systems
• Mainframe systems used magnetic tape reels
• Newer tape systems use tape cartridges or magnetic tape streamers for backing up data.

Internet Hard Drives

• Special web sites that provide users with free or low-cost storage
• Also called i-drive or online storage
• Advantage is low(or no) cost and the flexibility to access information from any location using any computer as long as you have an Internet connection
• Disadvantage is access time is often more, and there is some hesitation about storing sensitive data on these sites.

Solid-state storage

• These devices have no moving parts, so they are fast, require less power and reliable
• Tends to have less capacity, and costs more per byte
• Flash memory cards are used in notebook computers and digital cameras