Storage devices and media
What is data storage?
When we talk about ‘storing’ data, we mean putting the data in a known place. We can later come back to that place and get our data back again.
‘Writing’ data or ‘saving’ data are other ways of saying ‘storing’ data.
‘Reading’ data, ‘retrieving’ data or ‘opening’ a file are ways of saying that we are getting our data back from its storage location.
Backing storage (sometimes known as secondary storage) is the name for all other data storage devices in a computer: hard-drive, etc.
Why back up data?
Serial / Sequential Access
A serial (or sequential) access storage device is one that stores files one-by-one in a sequence.
Direct / Random Access
A direct (or ‘random’) access storage device is one that stores files so that they can be instantly accessed - there is no need to search through other files to get to the one you want.
Data storage capacity
Data storage capacity is measured in bytes (B).
A thousand bytes is known as a kilobyte (kB)
1,000B = 1kB
A million bytes is known as a megabyte (MB)
1,000,000B = 1MB
A thousand million bytes is called a gigabyte (GB)
1,000,000,000B = 1GB
A million million bytes is called a terabyte (TB)
1,000,000,000,000B = 1TB
Data access speeds
Access speeds are measured in bytes per second (Bps).
Slow devices have speeds measured in thousands of Bps (kBps).
E.g. a floppy disc can save/read data at a speed of 60kBps
Fast devices have speeds measured in millions of Bps (MBps).
E.g. a hard-drive can save/read data at a speed of 300MBps (5000 times quicker than the floppy!)
Different types of storage
Is it magnetic?
Magnetic storage media and devices store data in the form of tiny magnetised dots. These dots are created, read and erased using magnetic fields created by very tiny electromagnets.
In the case of magnetic tape the dots are arranged along the length of a long plastic strip which has been coated with a magnetisable layer (audio and video tapes use a similar technology).
In the case of magnetic discs (e.g. floppy disc or hard-drive), the dots are arranged in circles on the surface of a plastic, metal or glass disc that has a magnetisable coating.
Examples of magnetic storage
Fixed hard disk
Portable hard disk
Floppy hard disk
Fixed hard-drive (Magnetic)
Hard-drives have a very large storage capacity (up to 16TB). They can be used to store vast amounts of data. Hard-drives are random access devices and can be used to store all types of films, including huge files such as movies. Data access speeds are very fast.
A portable hard-drive is one that is placed into a small case along with some electronics that allow the hard-drive to be accessed using a USB or similar connection.
Uses of portable hard disks
A removable, portable, cheap, low-capacity (1.44MB) storage medium.
Floppy discs are random access devices used for transfer small amounts of data between computers, or to back-up small files, etc. Access times are slow.
Almost every PC used to have a floppy disc drive. These are obsolete now, having been replaced by higher capacity technology such as CD-ROMs, DVDs and USB memory sticks.
Magnetic tape is a large capacity, serial access medium.
Uses of magnetic tapes
Optical storage devices save data as patterns of dots that can be read using light. A laser beam is the usual light source.
The data on the storage medium is read by bouncing the laser beam off the surface of the medium. If the beam hits a dot it is reflected back differently to how it would be if there were no dot. This difference can be detected, so the data can be read.
Dots can be created using the laser beam (for media that is writable such as CD-Rs). The beam is used in a high-power mode to actually mark the surface of the medium, making a dot. This process is known as ‘burning’ data onto a disc.
Read-Only optical disks
Advantages and disadvantages of CD and DVD ROM's
CD-R & DVD-R
CD-Recordable (CD-R) and DVD-recordable (DVD-R) discs can have data burnt onto them, but not erased.
Uses of CD-R & DVD-R
CD-RW & DVD-RW
Advantages & disadvantages of using DVD-RW & CD-RW
Blu-ray is an optical disc format such as CD and DVD.
Stands for "Digital Versatile Disc Random Access Memory."