SEB070030 Tutorial 05

Magnetic disk

A flat rotating disc covered on one or both sides with some magnetically coated surface that can store encoded information. There are two main types of magnetic disk, which are the hard disk and the floppy disk. Hard disk spins much faster than floppy disk do and has a higher storage density.

The read/read head magnetizes parts of the surface to record information. It glides on a thin cushion of air above the disk and never actually touches the disk. Digital information is stored on magnetic disks in the form of microscopically small, magnetized needles, each of which encodes a single bit of information by being polarized in one direction (representing 1) or the other (representing 0).

Examples
Floppy disk, hard disk, removable cartridge

Advantages

• Disk drive can rapidly retrieve information from any part of a magnetic disk without regard for the order in which the information was recorded.
• Because of their random-access capability, disks are the most popular media for storage needs.

Optical disk

Optical disk is an electronic data storage medium that can be written to and read using a low-powered laser beam. It is flat, circular (usually polycarbonate). Data are stored in the pits (or bumps) in its flat surface — sequentially on the continuous, spiral track extending from the innermost track to the outermost track, covering the entire disc surface.

The data are accessed in the disc when a special material (often aluminium) is illuminated with a laser diode. The pits distort the reflected laser light, hence, most optical discs, characteristically have an iridescent appearance created by the grooves of the reflective layer.
Write-once optical discs commonly use an organic dye, and re-writable discs use phase change alloys.

Examples
Compact disk, bluray

Advantages

• Optical disc offers a number of advantages over magnetic storage media. An optical disc holds much more data. The greater control and focus possible with laser beams (in comparison to tiny magnetic heads) means that more data can be written into a smaller space.
• Optical discs are inexpensive to manufacture and data stored on them is relatively impervious to most environmental threats, such as power surges, or magnetic disturbances.

Flash memory

Flash memory is a type of eraseable, non-volatile memory chip. This technology is primarily used in 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; in early flash the entire chip had to be erased at once.

Advantages

• Flash memory costs far less than byte-programmable EEPROM.
• It is non-volatile that no power is needed to maintain the information stored in the chip.
• It offers fast read access and better kinetic shock resistance than hard disk.
• In memory cards, it is enormously durable that can withstand extreme of temperature, intense pressure and immersion in water.

Examples
Mini SD card, MP3 player, USB Flash Drive


Magneto-optical disc (MO)

A plastic or glass disk coated with a compound with special optical, magnetic and thermal properties. It uses magnetic fields for data storage and is capable for writing and rewriting data.

The disk is read by bouncing a low-intensity laser off the disk. Originally the laser was infrared, but frequencies up to blue may be possible giving higher storage density. The polarisation of the reflected light depends on the polarity of the stored magnetic field.

Examples
Diskette drive, Rewritable OD 230MB Data Tape Cartridge

Advantages

• CD-MO is read by a laser beam, which makes it more reliable than a hard disk or a floppy disk. However, a strong magnetic field can corrupt the stored data.
• Although optical, they appear as hard disk drives to the operating system but need no special filesystem.