Magnetic storage media and devices store data in the form of tiny magnetised dots. Theses 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.
In the case of magnetic discs, the dots arranged in circles on the surface of a plastic, metal or glass disc that has a magnetisable coating.
Hard drives have a very large storage capacity. 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.
Data is stored inside a hard-drive in rotating metal or glass disks.
Fixed Hard Drive
A hard drive built into the case of a computer is known as 'fixed'. Almost every computer has a fixed hard drive.
Fixed hard drives act as the main backing storage device for almost all computers since they provide almost instant access to files (random access and high access speeds).
Portable Hard Drive
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.
Portable hard drives allow very large amounts of data to be transported from computer to computer.
Many portable music players (such as an iPod) contain tiny hard drives. These miniature devices are just not much bigger than a stamp, but can still store over 100MB of data.
Magnetic tape is a large capacity, serial access medium. Because it is a serial access medium, accessing individual files on a tape is slow.
Tapes are used when large amounts of data need to be stored, but where quick access to individual files is not required. A typical use is for data back-up.
Tapes are also used in some batch processing applications.
Removable Magnetic Disks: Floppy Disk
A removable, portable, cheap, low capacity storage medium. Floppy disks are random access devices used for transferring 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 These are obsolete now, having been replaced by higher capacity technology such as CD-ROMs, DVDs, and USB memory sticks.
Removable Magnetic Disks: Zip Disk
A removable and portable storage medium, similar in appearance to a floppy disc, but with a much higher capacity.
Zip disks are random access devices which were used for data back-up or moving large files between computers.
Another obsolete storage device, zip discs were a popular replacement for floppy discs for a few years, but they never caught on fully before being superseded by cheaper media like CD-Roms and CD-Rs.
Removable Magnetic Discs: Jaz Disc
A removable and portable storage medium based on hard drive technology, with a large capacity.
Jaz discs are random access devices which were used for data back-up or moving large files between computers.
Discs were expensive to buy and not very reliable.
Like the zip disc, this system never really caught on and was superseded by far cheaper and more reliable and cheaper technology.
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 then 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 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 Discs
Read-only optical discs have data written onto them when they are manufactured. This data cannot be changed.
Compact Disc - Read only memory (CD-ROM) discs can hold around 800MB of data. The data cannot be altered (non-volatile), so cannot be accidentally deleted. CD-ROMs are random-access devices.
CD- ROMs are used to distribute all sorts of data: software, music, electronic books (e.g. an encyclopedia with sound and video.)
Digital Versatile Disc - Read Only Memory (DVD-ROM) discs can hold around 4.7GB of data (a dual-layer DVD can hold twice that). DVD-ROMs are random access devices.
DVD-ROMs are used in the same way as CD-ROMs but, since they can hold more data, they are also used to store high quality video.
High Capacity Optical Discs: Blu-Ray
Blu-Ray discs are a recent replacement for DVDs. A Blu-Ray disc can hold 25-50GB of data. Blu-Ray discs are random access devices.
Blu-Ray discs are used in the same way as DVD-ROMs but, since they can hold more data, they are also used to store very high quality, high definition video.
High Capacity Optical Discs: HD DVD
High-Density DVD discs can hold around 15GB of data. HD-DVD's are random access devices.
HD-DVD discs are used in the same way as DVD-ROMs but, since they can hold more data, they are also used to store very high quality, high definition video.
Recordable Optical Discs
Recordable optical discs can have data written onto them ('burnt') by a computer user using a special disk drive (a disc 'burner').
CD-R and DVD-R
CD - Recordable and DVD - Recordable discs can have data burnt onto them, but not erased. You can keep adding data until the disc is full, but you cannot remove any data or re-use a full disc.
CD-RW and DVD-RW
CD-ReWritable and DVD-ReWritable discs, unlike CD-Rs and DVD-Rs, can have data burnt onto them and also erased so that the discs can be re-used.
DVD-Random Access Memory discs are a type of rewritable DVD. They often come in a floppy disk style case to protect the disk.
DVD-RAM discs have a similar capacity to a normal DVD, holding 4.7GB of data. DVD-RAM discs are random access devices.
DVD-RAM discs are using in many camcorders.
The discs are much higher quality than normal DVD-RWs and can reliably store data for up to 30 years, This means that they are often used for video and and data back up and archiving.
Why Solid State?
The term 'solid state' essentially means 'no moving parts'.
Solid state storage devices are based on electronic circuits with no moving parts.
Solid state storage devices store data using a special type of memory called flash memory.
Flash Memory is a type of electronically erasable programmable read only memory (EEPROM). Flash memory is non-volatile but the data stored in it can also be erased or changed.
Flash memory can be found in many storage devices.
USB Memory Sticks
Memory sticks have made many other forms of storage almost obsolete.
Memory sticks are non-volatile, random access storage devices.
Each of these small devices has some flash memory connected to a USB interface. Plug it into your computer and it appears as a drive. You can then add files, erase files, etc. You can use it to move any type of file between computers.
Flash memory used to be very expensive, but in recent years it has become much cheaper and you can now buy a 16GB memory stick for just a few pounds.
Many of our digital devices require compact, non-volatile data storage. Flash memory cards provide this and come in a variety of shapes and sizes.
One of the most common formats used by digital cameras is the SD Card. The cards store digital images taken by the camera.
Mobile phones contain a subscriber identity module (SIM) card that contains the phone's number, the phonebook numbers, text messages, etc.
Many phones also have extra memory cards to store music, video, photos, etc.
Many credit cards, door entry cards, satellite TV cards, etc. have replaced the very limited storage of the magnetic strip with flash memory. This is more reliable and has a much larger storage capacity.
Cards with flash memory are called smart cards.