Storage Devices

Primary Storage - Is temporary storage that is quickly readable to the processor.
Secondary Storage - Is permanent storage for programs and other files that can be readily accessed by the computer, but at a slower speed than Primary Storage.

 Magnetic
Hard Disk Drive - The disk allows rapid direct access to large amounts of data. The disk is also known as a platter. The platter is coated with an emulsion of iron and cobolt oxide particles that act as tiny magnets. Binary data is stored by aligning these tiny magnets in one direction to represent a 0 and another direction to represent a 1. Binary data is recorded in concentric rings, or tracks, which are subdivided into sectors that hold a fixed number of bytes. To read the data stored on the hard disk, the read-write head is moved to the desired track and waits for the relevant information to pass beneath it. When data is passed from the computer to the hard disk and vice versa, a whole sector is read or written each time. The top and bottom of the platter may be used to store data. Usually they are stored within a plastic box to restrict damage upon impact. The greater number of platters the more storage is available.

Floppy Disk - A very old form of storage that is used as operating system boot disks.

Magnetic Tape - Magnetic tape consists of a thin reel of plastic. The tape is coated on one side with a material that can be magnetised to record the bit patterns that represent data. The cartridge is completely sealed apart from an access door to put the tape in contact with the recording heads in the tape drive. Magnetic tape is robust, so it can be useful to transfer large pieces of data from one system to another. Finding a particular piece of data on a tape can take a long time, because the tape has to be searched from the beginning to the end. Although it is a cost-effective way to store data that does not have to be accessed immediately.

Optical
CD ROM - Read Only Memory. The data is written on the discs using disc-mastering machinery that impresses pits into the continuous spiral track. The disc surface contains a track that is 3.5 miles long. The disc spins at 200-500 revolutions and minutes. The data bit is read by a laser beam onto a point in the reflective metal layer where pits are impressed.

CD-R - The discs can be written only once. Any region can be written on but it cannot be altered. Data bits are recorded by burning a pit in a thin layer of film of metal using a high powered laser. A laser with much less power is used to read the disc. The recording beam creates a spot of less reflective disc by inducing a chemical reaction between the gold layer and it's dye coating.

CD-RW - A CD re-write can be read and written to as many times as you like. CD-RW drives have a magnet and a laser. The laser is similar to the one in the CD ROM drive. The write operations uses a magnet and the laser. The read operation only uses the laser. The data tracks are arranged in sectors. To record or change data the laser heats up a precise spot on the disc to around 200 degrees. The magnet is then used to set the state of the data bit at this spot. The disc cools and fixes this data bit. CD-RW drives keep the read-write heads a safe distance from the disc surface to avoid scratching. The laser light is less powerful as to not heat up the disc, it is polarised as it reflects off the surface and the degree of polarisation indicates the value of the data bit.

DVD-ROM - Offer more storage space compared to CD's. CD's and DVD's store data in the form of microscopic pits, which represent binary ones and zeros. A DVD can store more data due to the length of the pit size. In a CD the pits have to be a minimum length of 0.8 micro meters, however on a DVD they only have to be 0.4 micro meters allowing for more pits to be placed on the disc. To read the smaller pits the DVD drive uses a laser with a smaller wavelength. The red laser diodes used in DVD drives are identical to those in Bar Code Readers. Some manufacturers use both sides of the DVD disc to allow for more storage.

DVD-R - Record data by burning spots in the dye to alter the amount of laser light reflected.

DVD-RAM - Is a rewritable format that has a built in error control and a defect management system, so it is considered better than the other DVD technologies for tasks such as storage, backup and archiving data. RAM is used in camcorders and set top boxes as the data is easily rewritten and erased.

DVD-RW - The RW format provides a rewritable optical disc. They are commonly used for backing up data and home DVD recorders. One benefit of using a rewritable disc is that if there were writing errors while recording data then the data can be erased and stored on the disc again. The recording layer is not an organic dye, but a special phase-change metal back and forth between crystalline phase and an amorphous phase.

Blu-Ray Disc - A BD disc is a high density disc which stores digital information, including HDBD disc allows for quicker transfers of data.

HD-DVD - Works the same as Blu-Ray DVDs. They have become less popular as more people choose to buy BD discs.

Solid State
USB Flash Drive - A flash drive consists of a small printed circuit board typically in a plastic or metal casing. Only the USB connector is shown from the casing. Most USB Flash Drives uses a standard connection to allow it to be connected to a port on a computer easily. Flash drives are a relatively dense form of storage; even the cheapest flash drive can store all the data of a dozen floppy disks. They are generally more robust than Hard Disk Drives.

Memory Card - Usually used for storage in digital cameras, hand-held and laptop computers, telephones, music players, and games consoles. Memory cards offer a high capability for re-recording, power-free storage. They are generally more robust than Hard Disk Drives.