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Hard disks were invented in the 1950s. They started as large disks up to 20 inches in diameter holding just a few megabytes . They were originally called fixed disks or Winchesters a code name used for a popular IBM product. They later became known as hard disks to distinguish them from Floppy Discs. Hard disks have a hard platter that holds the magnetic medium, as opposed to the flexible plastic film found in tapes and floppies.

A hard disk is not that different from a cassette tape. Both hard disks and cassette tapes use the same magneticway of recording Hard disks and cassette tapes also share the major benefits of magnetic storage -- the magnetic medium can be easily erased.

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3.5 and 2.5 Inch

3.5 inch

4 in × 1 in × 5.75 in (101.6 mm × 25.4 mm × 146 mm) = 376.77344 cm³. This smaller form factor is similar to that used in an HDD by Rodime in 1983, which was the same size as the "half height" 3½" FDD, i.e., 1.63 inches high. Today, the 1-inch high ("slimline" or "low-profile") version of this form factor is the most popular form used in most desktops.

2.5 inch

2.75 in × 0.275–0.59 in × 3.945 in (69.85 mm × 7–15 mm × 100 mm) = 48.895–104.775 cm3. This smaller form factor was introduced by PrairieTek in 1988;[ there is no corresponding FDD. It came to be widely used for HDDs in mobile devices (laptops, music players, etc.) and for Solid state drives, by 2008 replacing some 3.5 inch enterprise-class drives. It is also used in the Playstation 3 and Xbox 360video game consoles. Drives 9.5 mm high became an unofficial standard for all except the largest-capacity laptop drives (usually having two platters inside); 12.5 mm-high drives, typically with three platters, are used for maximum capacity, but will not fit most laptop computers. Enterprise-class drives can have a height up to 15 mm.Seagate released a 7mm drive aimed at entry level laptops and high end netbooks in December 2009

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NTFS

NTFS supersedes the FAT file system as the preferred file system for Microsoft's Windows operating systems. NTFS has several technical improvements over FAT and HPFS (High Performance File System), such as improved support for metadata, and the use of advanced data structures to improve performance, reliability, and disk space utilization, plus additional extensions, such as security access control lists (ACL) and file system journaling.

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Cassette Tape and Hard Drives

  • The magnetic recording material on a cassette tape is coated onto a thin plastic strip. In a hard disk, the magnetic recording material is layered onto a high-precision aluminium or glass disk. The hard-disk platter is then polished to mirror-type smoothness.
  • With a tape, you have to fast-forward or reverse to get to any particular point on the tape. This can take several minutes with a long tape. On a hard disk, you can move to any point on the surface of the disk almost instantly.
  • In a cassette-tape deck, the read/write head touches the tape directly. In a hard disk, the read/write head "flies" over the disk, never actually touching it.
  • The tape in a cassette-tape deck moves over the head at about 2 inches about 5.08 cm per second. A hard-disk platter can spin underneath its head at speeds up to 3,000 inches per second about 170 mph.
  • The information on a hard disk is stored in extremely small magnetic domains compared to a cassette tape's. The size of these domains is made possible by the precision of the platter and the speed of the medium.
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Capacity and How it Works

A typical desktop machine will have a hard disk with a capacity of between 250 gigabytes and 4 Terabytes. Data is stored onto the disk in the form of files. A file is simply a named collection of bytes. The bytes might be the ASCII codes for the characters of a text file, or a movie or a picture but no matter what it contains, however, a file is simply a string of bytes. When a program running on the computer requests a file, the hard disk retrieves its bytes and sends them to the CPU one at a time.

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It is a sealed aluminium box with controller electronics attached to one side. The electronics control the read/write mechanism and the motor that spins the platters. The electronics also assemble the magnetic domains on the drive into bytes and turn bytes into magnetic domains. The electronics are all contained on a small board that detaches from the rest of the drive.

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The platters - These typically spin at 3,600 or 7,200 rpm when the drive is operating. These platters are manufactured to amazing tolerances and are mirror-smooth.

The arm - This holds the read/write heads and is controlled by the mechanism in the upper-left corner. The arm is able to move the heads from the hub to the edge of the drive. The arm and its movement mechanism are extremely light and fast. The arm on a typical hard-disk drive can move from hub to edge and back up to 50 times per second -- it is an amazing thing to watch!

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In order to increase the amount of information the drive can store, most hard disks have multiple platters. This drive has three platters and six read/write heads:

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The mechanism that moves the arms on a hard disk has to be super fast and accurate . It can be constructed using a high-speed linear motor.

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Storing Data

Data is stored on the surface of a platter in sectors and tracks. Tracks are concentric circles, and sectors are pie-shaped wedges on a tracklike a floppy disc.

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A sector contains a fixed number of bytes -- for example, 256 or 512. Either at the drive or the operating system level, sectors are often grouped together into clusters.

The process of low-level formatting a drive establishes the tracks and sectors on the platter. The starting and ending points of each sector are written onto the platter. This process prepares the drive to hold blocks of bytes.

High-level formatting then writes the file-storage structures, like the file-allocation table, into the sectors. This process prepares the drive to hold files.

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Types of Hard Drives

Enterprise Class Hard DrivesThey are meant for 24x7 i/o functioning, can go up to 100% data usage at one go and are quite reliable. They are however, extremely expensive and require a high level of data integrity. The slightest chance of data corruption will result in huge data losses.Desktop Hard DrivesThey are what we generally used. They perform optimally for 8 hours a day, are cheaper than the former, and can access and modify backup files fast enough to prevent massive data loss in case of data corruption.Parallel Advanced Technology Attachment (PATA)These types of drives are also known as Integrated Drive Electronics (IDE) and Enhanced Integrated Drive Electronics (EIDE) drives. The labels relate to the type of interface that is employed to connect the disk drive to the CPU board. These drives utilize either a 40 or an 80 wire cable with a broad 40-pin connector. 40 wire cables are utilized in older and slower hard disks, whereas 80 wire cables are used in faster ones. Nowadays, these types of hard disks are being substituted by SATA hard disks. EIDE hard drives were introduced after some advancements in IDE hard disks, however, the term IDE refers to both IDE and EIDE disk drives.Serial ATA (SATA)These hard disks use a totally different connector than their PATA counterparts. Moreover, they also employ a different power adapter than IDE ones, though adapters are easily attainable. The main difference between a SATA and a PATA hard disk is that the former is thinner and purportedly has a faster data interface than the latter. Nevertheless, this speed dissimilarity is not distinguishable in PATA and SATA drives which have the same rpm rating. SATA drives are more efficient, and use less power than PATA ones.

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Continued

Small Computer System Interface (SCSI)These hard disks are similar to IDE hard drives. They also spin at a higher rate in comparison to IDE and SATA ones. IDE and SATA drives generally spin at 7,200 rpm, whereas SCSI ones spin at 10,000 to 15,000 rpm. Today, SATA drives featuring a speed of 10,000 rpm are also manufactured. The higher the rpm, faster is the data access, but it may also lead to a faster breakdown. SCSI hard disks need a controller that operates the interface between drives and the computer motherboard.Solid State Drives (SSD)These hard disks, unlike the other types, don't consist of moving components. Typical hard drives comprise a spinning magnetic disk that performs the function of data storage, but SSDs use semiconductors for this purpose. Since there are no moving components, these hard disks are much faster and less likely to break down than other drives. However, their price is a bit more than other hard disks.These are some hard drive types that are generally incorporated in desktop computers and laptops. I hope this article would have helped you with different types of hard drives.

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Manufactures

Western Digital

Seagate

Hitachi

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Sources

http://computer.howstuffworks.com/hard-disk.html

http://www.pcworld.com/article/127105/article.html

http://www.pcguide.com/ref/hdd/op/mediaSize-c.html

http://compreviews.about.com/cs/manhard/

http://www.buzzle.com/articles/hard-drive-types.html