1. Hard disks
A hard disk drive (HDD)[note 2] is a data storage device used for storing and retrieving digital information using rapidly rotating disks (platters) coated with magnetic material. On each individual disk, the data is stored in concentric tracks. For every track of data, there are corresponding tracks above and below it. The corresponding tracks form a cylinder. The number of cylinders is equal in number to the tracks on each surface of the disk. For each surface there is a read/write head, set on the tip of a small arm. These heads move in step with each other, traversing in and out over the spinning disk.
The total storage space on a hard disk is given by the following formula:
number of readable sides x nnumber of cylinders x sector per track x bytes per sector
RAID(Redundant Array of Inexpensive Drives): this storage device essentially carries multiple copies of data, on different hard disk drives. If one falls, the data can still be recovered from the others.
External Hard Disks: External hard disks are built in a separate case and are robust, small and lightweight. They can be easily and safely carried around and can usually be connected to any computer with a USB socket.
2. Optical discs
Compact disks (CDs): A typical CD stores around 700 MB of data on one side of the disc. Access time is slower than a hard disk.
A CD is a piece of plastic, about 4/100 of an inch or 1.2 mm thick. During manufacture, a pattern is etched onto the lower polycarbonate plastic layer. This pattern is a single, continuous, extremely long spiral track of data. This lower layer is covered with a thin, reflective aluminium layer. Then a thin acrylic layer is sprayed over the aluminium to protect it. The CD’s label is then printed onto the acrylic. CD drive has a laser beam in the read/write head, which can read the information on the disc.
Digital Versatile discs (DVDs): A digital versatile disc looks much the same as a CD. A single-sided layer DVD can hold up to 4.7GB on one side. In contrast, a dual-layer DVD can carry two layers of data on each of its two sides: this means it can hold up to 18 GB of video, audio or other information.
Types of CD and DVD:
- CD-R and DVD-R allow data to be written to them on one occasion only: after this, the data on them can only be read. This enable you to create your own music CDs and record TV programmes on DVD, CD-R and DVD-R are useful for backing up the hard disk on a computer, because once the data has been written it cannot be deleted or changed.
- CD-RW and DVD-RW can be written to repeatedly up to around 1000 times and can be used for continuously backing up data or archiving. Data recorded on them can be deleted or replaced by more up-to-date data.
- DVD-RAM may be sealed inside a cartridge and can be rewritten more than 100000 times. Data written to DVD-RAM is expected to last at least 30 years. DVD-RAM drives are used in video recorders, camcorders and computers.
- HD DVD is an optical disk format for storing digital information, similar to DVD but with sufficient capacity for high definition video and movies. Its development was supported by a group of manufacturers led by Toshiba. HD DVD was created to succeed DVD but found competition in Blu-ray, which is a rival format. In 2008, HD DVD marketing and development ceased.
- Blu-ray is a high definition DVD format supported by a group of manufacturers led by Sony. It is intended as a replaced for the current range of DVDs and is used for distributing HD material such as movies. A dual-layer Blu-ray can store 50 GB almost 10 times the capacity of a single-sided, single-layer DVD. It is the same size as a CD.
3. Flash memory
Flash memory is a type of EPROM. The memory can be reset quickly and easily so that it can be written to, read from and edited. It retains what has been stored without a power source.
A common use of Flash memory is for the basic input/ output system (BIOS) of your computer. On vitually every PC available, the BIOS makes sure the computer starts up as it should and that all the parts of the computer work together.
Memory cards are removable and are used in video games consoles, digital cameras, mobile phones, PDAs and other portable devices. PCs often have card slots. Memory cards can store up to 32 GB of data. There are many types of memory card, including Compact Flash (CF), Secure Digital (SD), mini and micro SD, xD Cards, Smart media and Multimedia cards.
Flash pen devices or memory sticks plug into the USB port on a computer. They are lightweight and fit easily into your pocket s they are a convenient way of moving data from one computer to another. They have a storage capacity of up to 32GB. They are used by teachers and lecturers to store presentations and other files, because they can be easily moved from one classroom to another. They are robust and not easily damaged, and are a relatively inexpensive medium.
4. Magnetic tape
A magnetic tape is wrapped onto a reel and provides a large amount of surface area. This abundance of surface means that a great quantity of information can be stored on a tape. But tape storage offers only serial access. So if you want to access data at the middle of the tape you must start at the beginning, and forward through the tape to reach the part you want. For this reason, accessing the data on a tape can be very slow, and so magnetic tape is mostly used for backing up data not for quick regular access.