2. TYPES OF EXTERNAL MEMORY
MAGNETIC DISKS
OPTICAL DISK
MAGNETIC TAPE
DISK DRIVE
3. 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.
5. 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.
6. 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.
8. 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.
10. 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.
