1. Magnetic Disk
Data is stored on a series of concentric circles which is called tracks and it is encoded as magnetic bit patterns. Data can be accessed directly by rotating the disk beneath an actuator for single disk or access arms for multi-disk. Data on single disks is organized in pie-like subdivisions while data on "disk packs" is organized in cylinders. Data sequences are placed on the same tracks of adjacent disks rather than on adjacent tracks on one disk. This minimizes the movement of the access arms and thus maximizing the access speed.
Examples:
1) Hard Disks rigid aluminum platters coated with magnetic oxide (MgO) whose high precision provides the highest storage capacities and quickest access rates of all magnetic media. This requires a contaminant-free environment. If contaminants do reach the disk surface a "crash" occurs where a scratch on the disk surface results in data loss.
2) Diskettes or "floppies" are made of flexible Mylar plastic coated with magnetic oxide (MgO). It can have single (SD), double (DD), or high densities (HD), and can be either single- or double-sided.
Advantages:
1) Hard disks have the advantage of being the fastest mass storage but are permanent (i.e. the storage disks can not be switched) and high capacity storage in that it can hold up to hundreds of gigabytes of information.
2) Disk drive can rapidly retrieve information from any part of a magnetic disk without regard for the other in which the information is recorded.
2. Optical Disk
Data is encoded on the disk surface by a laser beam rather than magnets to read and write bits of data on a reflective aluminium layer of the disk, either by burning holes (ablative method) or heating the surface until a bubble forms (bubble method). Data, text, audio signals and video images are stored as digitized patterns in respective frames. Data is read when the surface reflects light through a series of mirrors to a photodiode, in which it converts light to electrical signals.
Examples:
1) Analog data can be stored on read-only Video disks which has been typically used to store text, graphics, video images, and audio signals.
2) Digital data is stored on three different types of optical storage media:
a) Read-only storage disks includes CD-ROM and DVD-ROM
b) WORM (Write Once Read Many) technology is used in CD-R (compact disk recordable) which is popular for permanent storage, especially as a backup medium. WORM drives can not be used as secondary storage because data can be stored on them only once.
Advantages:
1) A transparent plastic disc surface protects the aluminium layer from routine physical damage while letting laser light through.
2) In terms of conveniences, an optical disk is a great way to store the software and data to be distributed or sold.
3. Flash Memory
Flash memory is non-volatile computer memory that can be electrically erased and reprogrammed. It is a technology that is primarily used for general storage and transfer of data between computers and other digital products. It is a specific type of EEPROM (Electrically Erasable Programmable Read-Only Memory) that is erased and programmed in large blocks.
Examples:
1) USB Flash drives are flash memory modules that plug into a USB port, serving as small, long lifetime, rapid access secondary storage. They serve as an efficient portable secondary storage devices.
2) CompactFlash is a new generation of high capacity secondary storage cards for digital cameras. The technology can provide sustained write speeds up to 750 KBps. It also features an intelligent power management scheme to reduce power consumption up to 100 per cent as well as reduced stand-by current requirements.
3) Memory stick is a flash memory card from Sony designed for handheld digital appliances such as cameras and camcorders. Transfer to a PC is made via a PC Card adapter.
Advantages:
1) Flash memory is non-volatile, which means that no power is needed to maintain the information stored in the chip and better kinetic shock resistance than hard disks.
2) It is enormously durable, being able to withstand intense pressure, extremes of temperature, and even immersion in water.
4. Magneto-optical Disk
In magneto-optical disk, lasers are used to read data as well as facilitate writing data. The data is stored magnetically in microscopic "magnetic domains". When the high-power laser heats the magnetic storage film the domains can be aligned in higher densities than on regular magnetic disks, thus giving higher storage capacity than hard disks. Data is retrieved by reflecting a low-power polarized laser beam off of the magnetic film. The polarization of reflected beam can be interpreted as binary data.
Example:
1) A MiniDisc (MD) is a magneto-optical disc-based data storage device initially intended for storage of up to 80 minutes of digitized audio. In the form of Hi-MD, it has developed into a general-purpose storage medium in addition to greatly expanding its audio roots.
Advantages:
1) Magneto-optical disks are removable and have removable gigabyte storage capacities, supposedly up to 20 GB
2) During recording, the laser power is increased so it can heat the material up to the Curie point in a single spot. This allows an electromagnet positioned on the opposite side of the disc to change the local magnetic polarization, and the polarization is retained when temperature drops.
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