In computing, external storage comprises devices that temporarily store information for transporting from computer to computer. Such devices are not permanently fixed inside a computer.
Semiconductor memories are not sufficient to provide the whole storage capacity required in computers. The major limitation in using semiconductor memories is the cost per bit of the stored information. So to fulfill the large storage requirements of computers, magnetic disks and optical disks are generally used.
Types of external storage
Magnetic storage
- Cassette tape
- Floppy disk
Optical storage
Optical media are the media that use laser light technology for data storage and retrieval.
Optical storage devices
CD
CD is an acronym for Compact Disc. The speed is much less than a hard disk. The storage capacity is approximately 700 MB depending on whether the data stored on it is compressed or not. Types of CDs include:
- CD-ROM: (Compact Disc Read Only Memory) It can only be read through the drive. And are usually manufactured by in bulk by a stamp type system.
- CD-R: (Compact Disc Recordable) was invented in the 1990s. Using CD-R, it is possible to write data once on a disc at home without the stamping equipment required for CD-ROMs. These are considered write once, read many disks.
- CD-RW: (Compact Disc Re-Writable) same as the CD-R but can be erased and reused. There is a limit on how many times a CD-RW can be written. Presently this limit is 1000 times. CD-RW drives are compatible with CD-ROM and CD-R.
DVD
DVD stands for Digital Versatile Disc. Its speed is much faster than CD but not as fast as hard disk. The standard DVD-5 technology has a storage capacity of 4.7 GB. The storage capacity changes with the standard used. Its storage capacity (6k GB) is much higher than a CD (700 MB). It is achieved by a number of design changes.
Blu-ray
Blu-ray storage capacity is up to 50 gigabytes (or even 100 GB) of data. The Blu-ray Disc (BD) is a digital optical disc format. It was originally created to take the place of the DVD format due to its expanded storage capacity. The name "Blu-ray" is derived from the use of a blue laser that is used to read the disc. This would be in contrast to the red laser used to read DVD Discs.
M-DISC
M-Discsare available as DVD or Blu-ray discs.
Solid State Storage
Flash memory is a form of solid state memory. It was invented in the 1980s by the Toshiba company. Flash memory is a particular type of EEPROM (Electrically Erasable Programmable Read Only Memory). It is a non-volatile memory. It retains the stored information without requiring a power source. It is called solid state memory because it has no moving parts. Flash memory is different from the regular EEPROM. In case of EEPROM data are erased one byte at a time which makes it much slower. On the other hand, data stored in flash memory can be erased in blocks. That is why it gets its name “flash memory†because the chip is organized in such a way that a block of memory cells can be erased at a single time or “flashâ€.
Advantages
- It has no moving parts and is therefore very durable and less susceptible to mechanical damage.
- It is small in size and light in weight. Hence it is extensively used in portable devices.
- Flash memory transfers data at a faster rate.
Disadvantages
- The cost of flash memory is high as compared to hard disk. Memory card (for example, CompactFlash) with a 192 MB capacity typically costs more than a hard drive with a capacity of 40 GB.
- The storage capacity of a flash memory is far less than a hard disk.
- Flash memory has a limited number of write cycles.
Flash memory devices
- Memory card: Memory cards are flash memory storage media used to store digital information in many electronics products. The types of memory cards include CompactFlash, PCMCIA, secure digital card, multimedia card, memory stick etc.
- Memory stick: Sony introduced memory stick standard in 1998. Memory stick is an integrated circuit designed to serve as a storage and transfer medium for digital data. It can store data in various form as text, graphics, digital images etc. transfer of data is possible between devices having memory stick slots. Memory sticks are available in various storage sizes ranging from 4 MB to 64 MB. The dimensions of a memory stick are 50Â mm long, 21.5Â mm wide and 2.8Â mm thick (in case of pro format). The transfer speed of memory stick is 160 Mbit/s.
Other devices
Compare external storage which need not have a permanent connection to a computer:
- external hard disk drives
- disk enclosures
See also
- Auxiliary memory
References
