Storage device: floppy disk drive (FDD) and hard disk drive (HDD)

This post is lesson 33 of 42 in the subject Computer Hardware

Storage devices used to store operating system, software, data, etc. Storage devices is also known as secondary memory or external memory. They are non-volatile memory. There are many storage devices such as floppy disk, hard drive, SSD, optical disc, USB, etc. This article will present the features of HDD (hard drive) and FDD (floppy drive).

secondary memory in computer

1. FDD (Floppy Disk Drive)

There is a limited capacity, up to 2.88 MB. Floppy disks typically use only 1.44 MB of capacity. FDD has slow access speed, limited capacity is the reason that floppy drives and floppy disks are almost no longer used.

FDD (Floppy Disk Drive)

Features of FDD

  • Size: 3.5”/5.25”
  • Capacity: 720KB, 1.44MB, 2.88MB
  • Conntection: cable with 34 pins
  • Rotation speed: 300 RPM

2. HDD (Hard Disk Drive)

HDD is compact, fast access speed, large storage capacity and durable. It is usually 3.5 inch or 2.5 inch of size.

HDD (Hard Disk Drive)

2.1. Physical structure of HDD

HDD consists of components:

  • Chassis
  • Spindle
  • Platter
  • Read/write head
  • Actuator
  • Control circuit
Structure of HDD

Chassis made of aluminum and plastic materials for protecting components of HDD.

Platter is typically made using an aluminum, glass or ceramic substrate, 2 sides coated with magnetic and protective layer, mounted on the same shaft, consisting of many magnetic disks stacked on top of each other.

Read/write head used to read/write data, each side of flatter has its own head.

Control circuit transmited data between mainboard and HDD, control read/write operation, control rotation speed.

Cache stored temporary data.

Actuator is head displacement motor.

2.2. Logical structure of HDD

Track is a circular strip of physical data blocks on platter.

Sector is an element on the track, each sector is 512 bytes in size.

Cylinder is a cylindrical intersection through the stack of platters in a disk, centered around the disk’s spindle.

Landing Zone is the pause position of the read/write head.

Cluster is a collection of sectors. Clusters are allocation units for data on various file systems (FAT, NTFS, etc.), where data mainly consists of files.

Track, cluster, sector in HDD

3. Working principle of HDD

The surface of magnetic disk is covered with a thin layer of magnetic substance. Initially, the magnetic particles have no direction, when the magnetic head is passed, the magnetic particles are arranged into directional particles.

During data recording: The electrical signal is fed into the magnetic head to transform magnetic particles into very small magnets. The direction of the magnetic particles depends on the input electrical signal is 0 or 1.

During data reading: The magnetic head scans the surface of the magnetic platter along tracks. At the intersection of the magnetic particles with head, the magnetic field changed and induced on the magnetic head an electric pulse. The electrical pulse is taken as a 0 or 1 signal.

working principle of HDD

4. Specifications of HDD


  • HDD’s data storage capacity
  • HDD capacity is equal to (number of bytes/sector) × (number of sectors/track) × (number of cylinders) × (number of read/write heads)
  • HDD capacity is usually in GB, TB (250 GB, 500 GB, 1 TB,…)

Rotational speed of magnetic disk

  • Rotational speed of magnetic disk to read/write data.
  • Usually denoted by RPM (Revolutions Per Minute) (5400rpm, 7200rpm, 10.000rpm, 15.000rpm,…)

Cache of HDD in the hard drive is responsible for temporarily storing data during the working process of the hard drive. The amount of cache memory in the hard disk drive is usually a few MB (4MB, 8MB, 16MB,…).

Connection are technical regulations that help to connect the HDD and the mainboard. HDD connection standards such as:

  • S-ATA
  • SCSI

Refer to the article Power pins and storage device pins on mainboard to learn the connection standards of the HDD.

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Posted by Vinh Le

Composer at ITLearningCorner.

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