This article is about external storage devices. For external storage in the context of data structures, see Reference (computer science)#External and internal storage.

In computing

Computing

Computing is usually defined as the activity of using and improving computer hardware and software. It is the computer-specific part of information technology...

, 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, optical disks are generally used.

Advantages of external storage

• External storage provides additional storage other than that available in a computer.
• Data can be transported easily from one place to another.
• It is useful to store software and data that is not needed frequently.
• External storage also works as data back up.
• This back up may prove useful at times such as fire or theft because important data is not lost.

Optical storage

Optical media are the media that use laser light technology for data storage and retrieval.

Optical storage devices

CD

CD stands for compact disk. The speed is much less than a hard disk. The storage capacity is 700 MB. Types of CDs include:

• CD-ROM: It is compact disk read only memory. It can be read only.
• CD-Recordables: It was invented in 1990s. Using CD-R it is possible to write data once on a disk.these are write once read many disks.
• CD-ReWritables: 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

A DVD is an optical disc storage media format, invented and developed by Philips, Sony, Toshiba, and Panasonic in 1995. DVDs offer higher storage capacity than Compact Discs while having the same dimensions....

DVD stands for digital versatile disk. Its speed is much fast 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 (4.7 GB) is much more higher than a CD(700 MB). It is achieved by a number of design changes.

Solid state storage

Flash memory

Flash memory

Flash memory is a non-volatile computer storage chip that can be electrically erased and reprogrammed. It was developed from EEPROM and must be erased in fairly large blocks before these can be rewritten with new data...

 is a solid state memory. It was invented in 1980s by Toshiba.
A flash memory is a particular type of EEPROM

EEPROM

EEPROM stands for Electrically Erasable Programmable Read-Only Memory and is a type of non-volatile memory used in computers and other electronic devices to store small amounts of data that must be saved when power is removed, e.g., calibration...

 (Electrically Erasable Programmable Read Only Memory). It is a no-volatile memory. It retains the stored information without requiring a power source.It is called as solid state memory because it has got on 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 extremely slower. On the other hand data stored in flash memory can be erased in blocks. That’s why it gets a name “flash memory” because the chip is organised in such a way that a block of memory cells can be erased at a single time or “flash”.

Advantages of flash memory

• It has got no moving parts. So it is durable and less susceptible to mechanical damages.
• 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.
• As erasing of information in blocks is possible, flash memories are useful in devices where frequent updating of data is required

Disadvantages of flash memory

• The cost of flash memory is high as compared to hard disk. Memory card (for example, CompactFlash) with a 192MB 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 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
  CompactFlash
  CompactFlash is a mass storage device format used in portable electronic devices. Most CompactFlash devices contain flash memory in a standardized enclosure. The format was first specified and produced by SanDisk in 1994...
  
  , PCMCIA, secure digital card
  Secure Digital card
  Secure Digital is a non-volatile memory card format developed by the SD Card Association for use in portable devices. The SD technology is used by more than 400 brands across dozens of product categories and more than 8,000 models, and is considered the de-facto industry standard.Secure Digital...
  
  , multimedia card, memory stick
  Memory Stick
  Memory Stick is a removable flash memory card format, launched by Sony in October 1998, and is also used in general to describe the whole family of Memory Sticks...
  
   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 4MB to 512MB. The dimensions of a memory stick are 50mm long, 21.5mm wide and 2.8mm thick (in case of pro format). The transfer speed of memory stick is 160 Mbps.

Other devices

Other external storage devices include:

• punched card
  Punched card
  A punched card, punch card, IBM card, or Hollerith card is a piece of stiff paper that contains digital information represented by the presence or absence of holes in predefined positions...
  
  s
• Zip disks
• microform
  Microform
  Microforms are any forms, either films or paper, containing microreproductions of documents for transmission, storage, reading, and printing. Microform images are commonly reduced to about one twenty-fifth of the original document size...
  
  s
• memory spot chip
  Memory spot chip
  The Memory Spot chip is an integrated circuit currently in development by Hewlett-Packard. The chip incorporates a central processing unit, random access memory and a wireless receiver, all bundled together in a device 1.4 or 2 mm²....
  
  s

Compare external storage which need not have a permanent connection to a computer:

• external hard disk drives
• disk enclosure
  Disk enclosure
  A disk enclosure is essentially a specialized chassis designed to hold and power disk drives while providing a mechanism to allow them to communicate to one or more separate computers. Drive enclosures provide power to the drives therein and convert the data sent across their native data bus into a...
  
  s