External memory refers to storage devices that can retain data when powered off, such as magnetic disks, optical disks, magnetic tapes, and RAID arrays. Magnetic disks use circular platters coated with magnetizable material and a head to read and write data via magnetic fields. Optical disks use light beams to read and write, while magnetic tapes have a magnetic coating and read/write head. RAID arrays provide redundancy across multiple disks for improved reliability.

1. External Memory

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Notre Dame University
Bangladesh
Submitted By:
Rup Chowdhury – 201120010
Submitted To:
Mostafiz Ahammed
Lecturer, Department of CSE
Notre Dame University Bangladesh

3. What is External Memory?
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External memory, also called "secondary memory" refers to a storage
device that can retain or store data persistently. They could be embedded or
removable storage devices.

4. Types of External Memory
External Memory
Magnetic
Disk
RAID
Optical
Memory
Magnetic
Tape

5. Magnetic Disk
A disk is a circular platter constructed of
nonmagnetic material called the substrate,
coated with a magnetizable material.
Traditionally the substrate used in a disk are
Aluminum or Aluminum alloy material.
But recently glass substrates are used in
magnetic disk.

6. Benefits of glass substrate
⁃ Improved surface uniformity
⁃ Reduction in surface detects, so reduced read/write errors
⁃ Lower flight heights
⁃ Better Stiffness
⁃ Better shock/damage resistance.

7. Read and Write Mechanisms
⁃ Recording and retrieval done via conductive coil called a head.
⁃ During read/write, head is stationary.
Write
• Current through coil produces magnetic field
• Pulses sent to head
• Magnetic pattern recorded on surface below
Read
• Magnetic field moving relative to coil produces current
• Coil is the same for read and write

8. RAID
RAID (redundant array of independent disks) is a way of storing the
same data in different places on multiple hard disks or solid-state
drives (SSDs) to protect data in the case of a drive failure. There are
different RAID levels, however, and not all have the goal of
providing redundancy.

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Optical Memory
These memories use light beams for its
operations and require optical drives for
its operations. These memories are used
for storing audio/video, backup as well as
caring for data. Read/write speed is
slower compared to hard disk and flash
memories.

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Magnetic Tape
The magnetic tape is a layer of plastic
coated with a magnetic documentation
medium. Magnetic tapes are used in most
organizations to save data files. Magnetic
tapes use a read-write mechanism. The
read-write mechanism defines writing data
on or reading data from a magnetic tape.