Hard Disk Structure In OS

Hard disk is secondary storage which stores the large amount of data. The Hard disk drive contains a dozens of disks.  These disks are also known as platters. These platters are mount over the spindle which rotates in any direction i.e. clockwise or anti-clockwise. Lets look at hard disk structure in OS.

Hard Disk structure in OS


  • The Platter is made of aluminum or iron oxide.
  • Platter diameter range is 1.8 inches to 5.25 inches
  • Each platter contains 2 surfaces and 2 Read/Write Head. One Read/Write head requires for one surface of the platter. So, other R/W head use for other surface to store the information’s.
  • Every platter holds the same no. of tracks.
  • Multiple platters increase the storage capacity.

R/W Head

  • R/W Heads moves forth and back over the platter surfaces to Read or Write the data on sectors.
  • Read/Write heads does not touch to platters surface. The data written over the platter surface is done through magnetic field. If R/W Head touches over the surface of platter then bad sectors may creates. Had disk may damage due to these bad sectors.


  • Circular areas of disk are known as tracks.
  • There may be more than a 1000 tracks on a 3.5 inch hard disk and sector size. Track Numbering start with zero at outermost track.


Tracks are further divided into number of small units; these small units are known as sectors. Sectors are the smallest physical storage units on disk. Size of each sector is almost always 512 Bytes. Sector Numbering start with 1 at outermost tracks.


All Corresponding tracks with same radius of all platters in the Hard disk are known as cylinders.  In simple words we say

“Each tack of all platters with same radius is called a cylinder”

So, Number of tracks in platter is always equal to number of cylinders. For example, a hard disk, where each platter contains 600 tracks then the number of cylinders will also be 600 in the hard disk.

Cylinder Numbering start with zero at outermost cylinder.


Cluster is also known as blocks. Group of sectors makes a cluster. There may be 64 or sectors in a cluster. These clusters are used by OS to Read/Write the data.

Disk Capacity

As we know there are number of platters in the hard disk. Each platter contains two R/W heads. There are number of cylinder/tracks in the hard disk. Each track is divided into number of sectors. And each sector has some size but mostly size of sector is 512 Bytes.

Question 01: Let suppose there are 8 platters in hard disk drive. As each platters has two surfaces so 16 surfaces will be there in hard drive. Therefore, required R/W head will also be 16. Suppose, there are 1,024 cylinders and 128 sectors in each track. The sector size is 512 bytes.Then

A). Calculate Disk Size?
B). How many number of bits required to represent the disk size?

Part A. Solution

Size of Hard Disk = Cylinder x Heads X Sectors x Sector-Size 
= 1,024 x 16 x 128 x 512 Bytes
=  210 + 24 + 27 + 29 Bytes = 230Bytes = 2GB

Part B. Solution

 As 2GB = 230 So, 30 bits are required to represent  2GB hard disk.

Question-02:  Find the size of Single platter if track size 1KB and there are 1024 cylinder? 

Solution: As there is a single platter So, Heads requirement will be 2.

Size of Platter = Cylinder x Heads X track size (because, Track size = Sectors x Sector-Size)
= 210 + 21 + 210 Bytes
=21+220 Bytes = 2MB

Question 03: Why does sector number addressing in CHS (Cylinder Head Sector) start at sector 1 and not 0?

Answer: For all translation modes, Cylinder (C) =0, Head (H) =0 and Sector (S) =1 is equivalent to Logical Block Address (LBA) =0. So the value of Sector is always 1.

Logical structure

Logical division of hard disk is generally divided into 5 basic terms which are

  • MBR (Master Boot Record)
  • DBR (DOS Boot Record)
  • FAT (File Allocation Tables)
  • Root Directory
  • Data Area



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