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Removable Storage and/or Disk Drives

All disks need a drive to get information off - or read - and put information on the disk - or write. Each drive is designed for a specific type of disk whether it is a CD, DVD, hard disk or floppy.

Often the term 'disk' and 'drive' are used to describe the same thing but it helps to understand that the disk is the storage device which contains computer files - or software - and the drive is the mechanism that runs the disk.

Digital flash drives work slightly differently as they use memory cards to store information so there are no moving parts.

Digital cameras also use Flash memory cards to store information, in this case photographs. Hand held devices use digital drives and many also use removable or built in memory cards.

Hard disk drive:

A hard disk drive (hard drive, hard disk, or disk drive) is a device for storing and retrieving digital information, primarily computer data.

It consists of one or more rigid (hence "hard") rapidly rotating discs (often referred to as platters), coated with magnetic material and with magnetic heads arranged to write data to the surfaces and read it from them.

Hard drives are classified as non-volatile, random access, digital, magnetic, data storage devices.

Introduced by IBM in 1956, hard disk drives have decreased in cost and physical size over the years while dramatically increasing in capacity and speed.

Hard disk drives have been the dominant device for secondary storage of data in general purpose computers since the early 1960s. They have maintained this position because advances in their recording capacity, cost, reliability, and speed have kept pace with the requirements for secondary storage.

External removable hard disk drives offer independence from system integration, establishing communication via connectivity options, such as USB.  Plug and play drive functionality offers system compatibility, and features large volume data storage options, but maintains a portable design.

The HDD installs in one of the 3-1/2 inch internal drive bays in the PC. It is secured by machine screws.

It is powered by a 4 conductor cable coming from the power supply.

Data to and from the motherboard is carried on a 40-pin IDE (Integrated Drive Electronics) cable.

Data is stored magnetically on multiple rigid disks that are stacked up like pancakes. Small arms with magnetic pickups move rapidly back and forth across the top and bottom surface of each disk in the drive. The sensors float just a few microns above the rotating disk surface and can read and write data at very high rates.

Most commercially available hard drives rotate at 5400 or 7200 RPM (revolutions per minute) which translates to 90 or 120 revolutions per second respectively. The data transfer rate from the drive to the motherboard is 33 Mbytes/second in bursts. Newer drives are capable of higher speeds up to 66 Mbytes/sec. To use this faster drive, the PC must have an ATA/66 interface that is capable of keeping up with it.

Floppy Disk:

A floppy disk is a disk storage medium composed of a disk of thin and flexible magnetic storage medium, sealed in a rectangular plastic carrier lined with fabric that removes dust particles. They are read and written by a floppy disk drive (FDD).

Floppy disks, initially as 8-inch (200 mm) media and later in 5.25-inch (133 mm) and 3.5-inch (89 mm) sizes, were a ubiquitous form of data storage and exchange from the mid-1970s well into the first decade of the 21st century.

It is powered by a cable with a 4-pin connector that comes from the power supply.

It transfers data to and from the motherboard by means of a 34 pin ribbon cable.

It stores data magnetically on a removable floppy disk. A pickup arm in the drive floats above the disk surface. The arm moves rapidly back and forth across the disk surface as a small magnetic sensor at the end of the arm reads and writes data on the rotating disk surface.

The first floppy disk was 8 inches in diameter, and was protected by a flexible plastic jacket. IBM used this size as a way of loading microcode into mainframe processors, and the original 8 inch disk was not field-writeable. Rewriteable disks and drives became useful. Early microcomputers used for engineering, business, or word processing often used one or more 8 inch disk drives for removable storage; the CP/M operating system was developed for microcomputers with 8 inch drives.

3 12-inch floppy disk were produced with a capacity of 720 KB, followed by what became the most common format, 1.44 MB. All disks had a rectangular hole which, if and only if obstructed, write-enabled the disk. 1.44 MB disks had another hole which identified them as being of that capacity.

Floppy disk size is often referred to in inches, even in countries using metric and though the size is defined in metric. The ANSI specification of 3 12-inch disks is entitled in part "90 mm (3.5 in)" though 90 mm is closer to 3.54 inches. Formatted capacities are generally set in terms of kilobytes and megabytes.

Compact Disks
  • The CDD installs in one of the external 5-1/4 inch drive bays in the front of the PC case. It is secured by machine screws. Some manufacturers offer special rail-like systems that mount on the CDD. These allow the drive to be removed from the PC without having to remove any screws.
  • Data is stored optically on the surface of the disk. A laser attached to an arm that moves back and forth across near the disk surface and sends light toward the disk surface which is coated with of a thin layer of aluminum. 
  • Smooth areas called a lands reflect the light back to a photo diode located near the laser. The reflected light is read as a 1. Areas called pits are where the aluminum has been removed. When the laser light hits these, it is scattered and very little is picked up by the photo diode. The absence of light is read as a 0. 
  • CDD have become the predominant removable storage media for PCs and can store 700 Mbytes of data. 
  • A 4-pin cable from the power supply plugs into the CDD and provides power to it. 
  • Data to and from the motherboard is carried on a 40-pin IDE (Integrated Drive Electronics) cable. 
  • There are two types of Compact Disk drives available for PCs.

    • CD-ROM (read only memory) is the older type. As the title implies it can only read CDs. It can read any standard CD and most CD-R type disks. It may be able to read some types of CD-RW disks too. A 24x CD-ROM unit costs about $30.
        • CD-RW (ReWritable) units can read and write CD-R and CD-RW type disks. It can also read standard CD type disks.
          A 24x10x40 CD-RW unit costs about $115. The 24x10x40 means the unit can write at 24x, re-write at 10x and read at 40x, where x is 150Kbytes/sec.


        DVD is an optical disc storage media format.

        Pre-recorded DVDs are mass-produced using molding machines that physically stamp data onto the DVD. Such discs are known as DVD-ROM, because data can only be read and not written nor erased.

        Blank recordable DVDs (DVD-R and DVD+R) can be recorded once using optical disc recording technologies and supported by optical disc drives and DVD recorders and then function as a DVD-ROM.

        Rewritable DVDs (DVD-RW, DVD+RW, and DVD-RAM) can be recorded and erased multiple times.

        DVDs are used in DVD-Video consumer digital video format and in DVD-Audio consumer digital audio format, as well as for authoring AVCHD discs. DVDs containing other types of information may be referred to as DVD data discs.

        DVD specifications created and updated by the DVD Forum are published as so-called DVD Books (e.g. DVD-ROM Book, DVD-Audio Book, DVD-Video Book, DVD-R Book, DVD-RW Book, DVD-RAM Book, DVD-AR Book, DVD-VR Book, etc.).

        Some specifications for mechanical, physical and optical characteristics of DVD optical discs can be downloaded as freely available standards from the ISO website. Also, the DVD+RW Alliance publishes competing DVD specifications such as DVD+R, DVD+R DL, DVD+RW or DVD+RW DL. These DVD formats are also ISO standards.

        The official DVD charter documents specify that the basis of the DVD name stems from the term digital versatile disc. Usage in the present day varies, with Digital Versatile Disc, Digital Video Disc, and DVD being the most common.

        DVD was originally used as an initialism for the unofficial term digital videodisc.

        There are two types of DVD drives that typically go into PCs.
        • DVD-ROM (Read Only Memory) can read DVDs and CDs. 
        • DVD-RAM (Random Access Memory) units can read and write DVDs. They can also read CDs.

        Blu-ray Disc:

        In 2006, two new formats called HD DVD and Blu-ray Disc were released as the successor to DVD. HD DVD competed unsuccessfully with Blu-ray Disc in the format war of 2006–2008. 

        A dual layer HD DVD can store up to 30GB and a dual layer Blu-ray disc can hold up to 50GB

        A high-definition television and appropriate connection cables are also required to take advantage of Blu-ray disc. Some analysts suggest that the biggest obstacle to replacing DVD is due to its installed base; a large majority of consumers are satisfied with DVDs. The DVD succeeded because it offered a compelling alternative to VHS.



        1. Excellent material..thanx 4 taking so much pain..its really very helpful!!


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