TYPES OF EXTERNAL MEMORY MAGNETIC DISKS OPTICAL DISK MAGNETIC TAPE DISK DRIVE
MAGNETIC DISK• Magnetic disk is a circular platter constructed of nonmagnetic material, called the substrate , coated with a magnetisable material.• Traditionally aluminum or aluminum alloy were used as substrates but recently glass substrates have been introduced.• Data are recorded on and later retrieved from the disk via a conducting coil named the head; in many system, there are two heads, a read head and a write head.• During a read or write operation, the head is stationary while the platter rotates beneath it.
OPTICAL DISK CD – Compact disk. A non-erasable disk that stores digitized audio information and can record more than 60 minutes of playing time. CD-ROM – Compact disk Read Only Memory. A non- erasable disk used for storing computer data of up to 650mb. CD-R – CD recordable. Similar to a CD-ROM. The user can write data only once. CD-RW – CD Rewritable. Similar to CD-ROM. The user can erase and rewrite to the disk multiple times.
DVD- digital versatile disk. A technology for storing large volume of video or other digital data of up to 4.7Gb. DVD-R – DVD recordable. The user can write to disk only once. DVD-RW – DVD rewritable. The user can erase and rewrite data to the disk multiple times.
MAGNETIC TAPE• Tape system use the same reading and recording techniques as disk system.• The medium is flexible polyester tape coated with magnetizable material.• Today, virtually all tapes are housed in cartridges.• Unlike the disk, the tape is in motion only during a read and write operation.
DISK DRIVE Hard disk drives were introduced in 1956 as data storage. In hard disk information is written to and read from a platter as it rotates past devices called read-and-write heads that operate very close (often tens of nanometers) over the magnetic surface. The hard drive`s electronics control the movement of the actuator and the rotation of the disk, and perform reads and writes on demand from the disk controller. The capacity of hard disk drives is in megabytes, gigabytes or terabytes.