Hard Disk Drive (HDD) Secondary Memory

Primary memory, like RAM, is limited and volatile, losing data when power is off. Secondary memory solves this by providing large, permanent storage for data and programs.

A hard disk drive (HDD) is a fixed storage device inside a computer that is used for long-term data storage. Unlike RAM, HDDs retain data even when powered off, making them essential for operating systems, applications, and files.

Secondary storage can be fixed (HDDs) or removable (USB drives, external HDDs), offering flexibility for users. This article explores HDDs, their working, advantages, and role in modern computing.

What is a Hard Disk Drive (HDD)?

A Hard Disk Drive (HDD) is a storage device that uses magnetic technology to store and retrieve digital data. It consists of spinning platters coated with a magnetic material and a read/write (R-W) head that moves to access data.

The disk is divided into tracks, and each track is further divided into sectors. Interestingly, even though the outer tracks are larger than the inner tracks, they contain the same number of sectors. This happens because the inner tracks have higher storage density, meaning the data is packed more tightly, while the outer tracks have more space between bits.

Some space in each sector is used for formatting, so the actual storage available is slightly less than the total capacity. The R-W head moves over the rotating disk to read and write data. To access a specific memory location, the head must be positioned correctly, which affects the speed of data retrieval.

Here are some key terms to understand HDDs better:

1. Seek time - The time taken by the R-W head to reach the desired track from its current position.
2. Rotational latency - Time taken by the sector to come under the R-W head.
3. Data transfer time - Time is taken to transfer the required amount of data. It depends upon the rotational speed.
4. Controller time - The processing time taken by the controller.
5. Average Access time - seek time + Average Rotational latency + data transfer time + controller time.

Note: Average Rotational latency is mostly 1/2*(Rotational latency). 

In questions, if the seek time and controller time are not mentioned, take them to zero. If the amount of data to be transferred is not given, assume that no data is being transferred. Otherwise, calculate the time taken to transfer the given amount of data. The average rotational latency is taken when the current position of the R-W head is not given. Because the R-W may be already present at the desired position or it might take a whole rotation to get the desired sector under the R-W head. But, if the current position of the R-W head is given then the rotational latency must be calculated. 

For Example - 

Consider a hard disk with: 

• 4 surfaces
• 64 tracks/surface
• 128 sectors/track
• 256 bytes/sector

What is the capacity of the hard disk? 

• Disk capacity = surfaces * tracks/surface * sectors/track * bytes/sector
  Disk capacity = 4 * 64 * 128 * 256 
  Disk capacity = 8 MB

The disk is rotating at 3600 RPM, what is the data transfer rate? 

• 60 sec -> 3600 rotations
  1 sec -> 60 rotations 
  Data transfer rate = number of rotations per second * track capacity * number of surfaces (since 1 R-W head is used for each surface) 
  Data transfer rate = 60 * 128 * 256 * 4 
  Data transfer rate = 7.5 MB/sec

The disk is rotating at 3600 RPM, what is the average access time? 

• Since seek time, controller time and the amount of data to be transferred is not given, we consider all three terms as 0.
  Therefore, Average Access time = Average rotational delay 
  Rotational latency => 60 sec -> 3600 rotations 
  1 sec -> 60 rotations 
  Rotational latency = (1/60) sec = 16.67 msec. 
  Average Rotational latency = (16.67)/2 
  = 8.33 msec. 
  Average Access time = 8.33 msec.

Another example: GATE IT 2007 | Question 44 

History of Hard Disk Drive (HDD)

Here is the Table of History of Hard Drive Disk:

Why do computers need Hard Disks?

Hard discs are used to install operating systems, programmes, and other storage devices, they are also used to save documents. Computer users would be unable to save their files or documents on their computers because HDD maintain data after being turning off of the computer. This is why every computer need at least one storage device to permanently save data for as long as it is needed.

How HDDs Work?

Most basic hard drives are made up of numerous disk platters, which are circular disks composed of aluminium, glass, or ceramic that are arranged around a spindle inside a sealed chamber

• Platters spin at high speeds (typically 5,400 to 15,000 RPM).
• Read/Write heads move to the correct track and position themselves over the sector containing data.
• Magnetic signals are written to or read from the spinning platters.
• The disk controller processes data and transfers it to the computer.

The platters rotate at up to 15,000 rotations per minute by the motor. A second motor regulates the location of the read and write heads that magnetically record and read information on each platter as the platters rotate. The compartments that store the data can be spread out all over the hard disk.

Hard Disk Drive Storage Capacity

• Three storage options 16 GB, 32 GB, and 64 GB. This is one of the lowest HDD storage space ranges and is often seen in older and smaller devices.
• There are two sizes 120 GB and 256 GB. This category is commonly regarded as an entry-level for HDD devices like desktops.
• 500 GB, 1 TB, and 2 TB are available. HDD storage of 500 GB or more is often deemed adequate for the average user. With this much capacity, users can most certainly save all of their music, photos, videos, and other information. Individuals with large-file games should find 1 TB to 2 TB of HDD capacity adequate.
• More than 2 TB of storage. Anything with more than 2 TB of HDD space is appropriate for users who work with high-resolution files.
• Recently, the highest capacity of HDD is 20 TB.

Hard Disk Drive Components and Form Factors

HDD has the aluminium components:

• Platters: Magnetic platters are disk-like in shape and used to store data. Typically, they are made from glass or aluminium and laid down on a spindle where layers of such disks are kept one after the other.
• Spindle: The spindle maintains the platters in place and rotates them as required. The Revolution Per Minute(RPM) rating controls how fast data can be written to and read from the hard drive.
• Actuator Arm: The actuator, also known as the head actuator, is a tiny motor that controls the movement of the read/write haed and monitors data transfer between the platters. It is in charge of ensuring that the read/write heads are always in the proper direction.
• Read/write Head: The read/write arm controls the movement of the read/write heads, that perform the actual reading and writing on the disc platters.

HDD the represents factors:

• 3.5 inches: Among the drive types that are common for desktops, 3.5-inch is the most commonly used form factor, it represents the approximate diameter of the platter within the drive enclosures
• 2.5 inches: These are used in laptops and other portable devices and they also used to represent the approximate diameter of the platter within the drive enclosures
• 1.8 inches: The 1.8-inch drives are generally employed in extremely light and thin devices like tablets and some MP3 players with large storage capacity
• Enterprise Form Factor: Some HDDs are designed to be used in corporation environments and are available in larger form elements which include 3.5-inch and a pair of.5-inch but with one-of-a-kind specs optimized for excessive overall performance, reliability, and durability in server and garage structures.

Features of HDD

• Non-Volatile: Data remains stored even when the power is off, making it ideal for long-term storage.
• High Capacity: Can store large amounts of data, with modern HDDs reaching terabytes in size.
• Relatively Slow: Slower than RAM, with access times in milliseconds, making it better for storage than frequent data access.
• Mechanical Parts: Contains spinning disks and moving read/write heads, which can wear out or get damaged.
• Cost-Effective: Cheaper than SSDs, especially for high-capacity storage.
• Reliable: Despite mechanical parts, HDDs are durable and widely used for enterprise storage.

Some Common HDD Errors

• Electrical Error: If the hard disc pois were on but cannot read or write data, one or more of its components has most likely failed electrically.
• Logical Failure: Logical failure occurs when the hard disk's software is hacked or stops working properly. All types of data corruption may lead to logical failure.
• Disk Failure: Disk failure ococcurshen the hard disk is not working properly and may lead to data loss.
• Disk Full: One more common issue is The isk full Issue which occurs when there is no more space on the disk to store data.
• Bad Sector: Bad sector failure occurs when the magnetic surface on a hard disk's platter is displaced, making a specific area of the platter inaccessible.
• Firmware Failure: Firmware failure occurs when the software that performs maintenance duties on a disk becomes corrupcorrupted

What are External HDDs?

External hard drives can be used as a portable data backup device or to increase a computer's storage capacity. Through connectors like USB 2.0, USB-C, or External SATA (eSATA), external disks can be connected to a computer or other device. Additionally, compared to internal HDDs, external hard drives may transport data more slowly.

In addition to being able to increase a system storage capacity, an external hard drive also has the benefit of being portable. Users are able to physically carry their stored data on numerous devices with them wherever they go.

Advantages vs. Disadvantages of HDD