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- 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, 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 devices provides additional storage other than that available in 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.
Types of external storage
Optical media are the media that use laser light technology for data storage and retrieval.
Optical storage devices
CD stands for Compact Disc. The speed is much lesser than a hard disk. The storage capacity is 700 MB. Types of CDs include:
- CD-ROM: It is Compact Disc 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 disc. 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 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 (4.7 GB) is much higher than a CD (700 MB). It is achieved by a number of design changes.
Solid state storage
Flash memory is a solid state memory. It was invented in 1980s by Toshiba. A flash memory is a particular type of EEPROM (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 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 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 no moving parts and is therefore very 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, 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 4MB to 64GB. 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 Mbit/s.
Other external storage devices include:
Compare external storage which need not have a permanent connection to a computer: