What is HDD (Hard Disk Drive)?

HDD (Hard Disk Drive) is a storage device that uses a series of magnetic coated rotating disks called platters to store data or programs.

What is HDD (Hard Disk Drive)?

What is Hard Disk/Fixed Disk?

In daily use, the terms hard disk or hard drive can be used interchangeably because of the disk and drive mechanism to form the same entity.

A typical disk platter drive rotates at speeds up to 3600 rpm, and the read/write heads float on an air cushion 10 to 25 million inches thick, so never contact the recording surface.

To prevent air pollutants from entering and interfering with these tolerances, the entire disk drive is hermetically sealed.

Hard drives can have large amounts of storage space from megabytes to gigabytes and even terabytes.

Also, the larger the disk, the more important the strategy you use for backup.

Hard drives are reliable, but they often fail at the most inopportune moment.

These are a non-volatile device that protects information even in energy loss using a Digital Magnetic Recording system.

There are different standards interfaces for connecting a hard drive to the computer. The most common interfaces are ATA/IDE (Integrated Drive Electronics), SCSI used in servers, SATA, standardized in 2004 and dedicated FC for servers.

As shipped from the factory, a hard drive does not come with an operating system such as Windows, Linux.

Before we define a low-level format, one or more sections, we need to set up a format that can be understood by our system.

There is another type of disk called Solid State that uses a particular type of memory used to store information with Semiconductors.

The use of this disc class was often limited to supercomputers due to their high price, but cheaper low-capacity drives are now available for use on personal computers.

Therefore, Track Cache is a solid-state memory, RAM type, inside the solid-state hard disk.


Initially, hard drives were removable, but today they are all typically added in a fixed box.

The first hard drive released in 1956 was the IBM 350 Model 1, available with the Ramac I computer, weighing a ton and its capacity was 5 MB.

This hard drive, larger than an existing refrigerator, was still working with vacuum valves and required a separate console for use.

Its greatest value was that the time required for access was relatively constant between some memory locations, unlike the given magnetic tapes, it was necessary to wind up and loosen the rollers until we found the desired data with very different access times for each information.

The first technology applied to hard drives was relatively simple. It involved coating a metal disc with magnetic material, which was later divided into sectors, formed in concentric tracks.

The magnetic head magnetized small parts of the hard drive using the binary code, which resets the information and ones. Bits or binary numbers recorded in this way can remain intact for years.

Initially, each bit had a horizontal arrangement on the magnetic surface of the disk, but it was discovered how to record information more compactly.

The merit of the French Albert Fert and German Peter Grünberg was the discovery of the phenomenon known as the giant magnetoresistance, allowing to create more precise reading and writing heads and to further compress the bits on the surface of the hard disk.

These discoveries, made independently by these researchers, led to dramatic growth in storage capacity on hard drives, increasing by 60% annually in the 1990s.

In 1992, 3.5-inch hard drives were 250 MB, while 10 years later their storage capacity exceeded 40GB. Today, there are disks containing more than one terabyte in daily use.

Also, in 2005, the first mobile phones with Hard Disk were introduced by Samsung and Nokia.

HDD Components

The components inside an HDD are;

  • Disk plates on which the data are recorded.
  • Reading and writing head.
  • The engine that turns the disk plates.
  • The electromagnetic component that moves the head.
  • Cache.
  • Electronic control circuit.
  • Moisture protective bag.
  • Outer box component to protect from dirt and dust.
  • Vent hole.
  • The screw-type component is also Torx.


The features to consider when purchasing a Hard Disk are:

   Average Access Time

The average time it takes for the head to reach the desired sector on the disk surface is the sum of Average Search Time, Read/Write Time and Average Delay time.

   Average Call Time

The average time it takes for the head to reach the desired track is half the time it takes to go from the peripheral to the most central path of the disc.

   Read/Write Time

The average time it takes for the disc to read or write new information. It depends on the amount of information you want to read or write, the block size, the number of laps, the time per turn, and the number of sectors per track.

   Average Latency

The average time it takes for the head to be in the desired sector is half the time it takes to spin a full disk.

   Return Speed

It is the revolutions of the disk plates per minute. The higher the rotation speed, the lower the average delay time.

   Transfer Rate

It is the speed of transferring information to the computer after the head is positioned on the right track and in the industry.


A RAM in the hard drive is memory. Semiconductor hard drives use certain types of memory created with semiconductors to store information.

   Socket Type

It is an interface communication medium between IDE/ATA, SCSI, SATA, USB, Firewire, SAS between hard drive and computer.

Currently, new generation drives use Perpendicular Recording (PMR) technology, which provides higher storage density.

There are also disks called GP (Green Power) that use energy more efficiently.

SSD disks, which are new storage types, are very fast since they do not have moving parts and consume less energy.

Physics Structure

Inside a disk, there are one or more plates, which are concentric discs and all rotate simultaneously.

The head, which functions as a reading and writing device, is a vertically aligned set of arms that move in or out simultaneously.

At the end of these arms are read/write heads that can read both the inner and outer areas of the disc thanks to the movement of the head.

Each tray has two faces and a read/write head is required for each face.

In reality, each of the arms is double and contains 2 heads, one for reading the top face of the plate and the other for reading the bottom face.

Therefore, there are 8 heads for reading 4 plates, but for commercial reasons, not all faces of the disks are always used, and there are hard drives with an odd number of heads or disabled heads.

The read/write heads never touch the disc, but they are very close to one-third of a millimeter due to a very thin layer of air that forms between the plates and the plates when they rotate.

If any of the heads touch the surface of a plate, it will cause a serious scratch, depending on how quickly the plates turn.


The disk uses a plate, face, head, trace, cylinder, and sector concepts as addressing:

  • Plate: Each of the disks inside the drive.
  • Face: It is the two sides of a plate, upper and lower.
  • Head: It is the end device that writes and reads.
  • Track: A circle within a face.
  • Cylinder: These are vertically aligned circles consisting of several parts.
  • Sector: Each section of a track. The sector size is not fixed, the current standard is 512 bytes.

In the past, the number of sectors per track was constant, which was significantly wasted, because more sectors from the inside can be stored from the outside.

Thus, ZBR technology, which increases the number of sectors on the outer tracks and uses the hard disk more efficiently, has emerged.

The first addressing system used was CHS because any data on the disk could be placed with these three values.

Later, a simpler system called LBA was created, which consists of segmenting the entire disk and assigning a unique number to each.

Connection Types

A solid disk contains the SATA, IDE, SCSI, or SAS different connection types they have with the motherboard.

   IDE Connection Type

Mass storage devices such as IDE or ATA, hard drives and ATAPI (Advanced Technology Attachment Packet Interface) are the main standard for versatility and quality.

   SATA Connection Type

It is the new connection standard that uses a serial bus for data transmission.

It is noticeably faster and more efficient than the IDE and has three versions;

  1. SATA1: The transfer speed is 1.5GB.
  2. SATA2 The transfer speed is 3GB.
  3. SATA3: The transfer speed is 6GB.

   SCSI Connection Type

They are hard drives with large storage capacity.

The average access time can reach 7 milliseconds and the sequential information transmission rate can theoretically reach 5 Mbps on Standard SCSI disks, 10 Mbps on Fast SCSI disks, and 20 Mbps on Wide Speed ​​SCSI disks (SCSI-2).

The SCSI controller can process up to 7 SCSI hard drives with a daisy chain connection. Unlike IDE disks, they can work asynchronously than the microprocessor, making them faster.

   SAS Connection Type

Although it continues to use SCSI commands to interact with SAS data devices, the successor of parallel SCSI is the serial data transfer interface.

It increases speed and allows for quick connection and disconnection.

One of the main features is to increase the transfer rate by increasing the number of connected devices, that is, to manage a fixed transfer rate for each connected device, as well as terminate the 16 device limitation in SCSI.

In addition, the connector is the same as in the SATA interface and allows the use of these hard drives for applications that require less speed, saves costs.

Therefore, SATA drives can be used by SAS controllers, but not vice versa because the SATA controller does not recognize SAS drives.


The oldest form factor of hard drives inherited their size from floppy drives.

They can be installed in the same chassis, and therefore form factor hard drives are popularly called FDD types in floppy drives.

Form factor compatibility continues to be 8.89 cm (3½ inches), even after other smaller floppy disk types are removed.

   8 inches

In 1979, Shugart Associates released the first form factor of the same size and compatible with the 8-inch interface of floppy drives of the same size.

   5,25 inches

This form factor uses the same size and maximum 5¼ inch FDD height, a maximum size of 82.5 mm by Seagate hard drives in 1980.

Most 120mm optical drive (DVD/CD) models use a 5¼ half-height form factor size but are also used for hard drives.

The Quantum Bigfoot model is the last model used in the late 1990s.

   3,5 inches

This form factor is first used by Rodine hard drives, which are the same size as drives 3½, 41.4 mm high.

   2,5 inches

This form factor was introduced by PrairieTek in 1988 and is completely different from the size of floppy drives.

This is often used by mobile computer hard drives and replaced 3.5-inch drives in 2008.

Today, this disk size is more common in laptop drives.

   1,8 inches

This form factor was introduced by Integral Peripherals in 1993 and was concerned with the ATA-7 LIF in the specified dimensions, and its use in digital audio players increased significantly.

The original variant is from 2GB to 5GB and fits in a PC expansion card slot. They are normally used on iPods and MP3-based hard drives.

   1 inches

This form factor was introduced in 1999 by IBM and Microdrive in accordance with compact flash type 2 slots. Also, Samsung uses the same factor as 1.3 inches.

   0,85 inches

Toshiba announced this form factor on January 8, 2004, for use in mobile phones and similar applications, including an SD/MMC slot compatible with video and HDD optimized for 4G micro mobile storage.

Naming disk sizes in inches does not normally describe existing products but indicates the relative size of the disc for the sake of historical continuity.

Durability and Fault Tolerance

Due to the extremely narrow space between the HDD heads and the surface, an accident of the read/write heads causes data loss when the head scratches the disc surface.

These accidents may include electronic malfunction, a sudden interruption in the power supply, physical shocks, wear and tear corrosion, or poorly produced heads or other causes.

The axis of the HardDisk system depends on the air pressure inside the enclosure to support the heads and their correct height as the drive rotates. The hard drive requires a certain air pressure line to function properly.

Connection and pressure to the external environment usually occur through a small hole in the housing with a filter inside.

When the air pressure is too low, there may not be enough space between the head and the disc surface, which can result in data loss. Therefore, discs capable of operating at a height of about 3 kilometers are required.

In modern aircraft, the pressure height is normally 2.6 KM, so it has a pressure cabinet. So you can safely use your storage devices on flights.

Modern devices include temperature sensors and are adjusted according to environmental conditions. Ventilation holes are visible on all disks. Normally there is a sticker with them to warn the user not to cover the hole.

The air inside the disc is constantly in motion to be swept away by the friction of the plate.

This air passes through an internal recirculation filter to remove any contaminants, a particle, or a chemical component that has somehow entered the device, through a particle during normal operation.

For a long time, too high humidity can wear the heads and plates.

For large magnetism-resistant (GMR) heads, in particular, a minor event due to contamination results in temporary head overheating due to friction with the disc surface and may result in data read up to the head for a short time.

The electronic components of the HDD control the movement of the actuator and the rotation of the disc, and perform the read and write operations required by the controller.

The firmware can efficiently program read and write operations to the surface and reassign failed sectors.


The technological resources and technical knowledge required for the development and production of modern devices have been produced by a group of large companies Seagate, Western Digital, Samsung, and Hitachi since 2007, more than 98% of the hard drives in the world.

Fujitsu continues to produce portable drives and server drives, but in 2001 it stopped making disks for desktop computers, the rest were sold to Western Digital.

Toshiba is one of the leading manufacturers of 2.5-inch and 1.8-inch laptop hard drives. ExcelStor is a small hard drive manufacturer.

The first victim in the PC market Computer Memories Inc/CMI After an incident involving a 20 MB faulty device drive in 1985, CMI’s reputation never recovered and left the hard drive market in 1987.

Another major failure was MiniScribe, which went bankrupt in 1990 after it was discovered that they were a fraud and inflated the number of sales for several years.

Micropolis was able to last until 1997, and the newcomer JTS lasted only a few years and disappeared in 1999, then tried to make hard drives in India.

Rodime was a major manufacturer in the 1980s as well, but in the midst of restructuring, it stopped recording in the 1990s and focused on licensing technology. It also has several patents related to the 3.5-inch format.

Damage Symptoms

If you do not have software that determines the failures of an storage, there are some indications that the disc is about to fail.

  • Irregular crashes or freezes, especially when the operating system starts up.
  • When copying a file, the process is interrupted or canceled altogether.
  • File or folder names that change automatically.
  • Meaningless deletion of files.
  • The process takes too long when you want to open a folder or file.
  • Sizzling sounds from the device.

Symptoms of sounds from the device are very important. This is physically a guarantee of the possibility of your device to be damaged, or it will indicate the failure state in a short time. Since SSDs used today are not mechanical type, this symptom is not considered.

SMART technology, which means “Self Monitoring Analysis and Reporting Technology” of modern disks, helps to detect and display faults.

There are several applications that read SMART data and show device status.

If your device’s SMART feature reports a problem, it is recommended that you back up your data to an external device immediately.

In addition, Bad Sector may also occur on an HDD, which not only prevents data saving and reading but also makes your device completely unusable. So you can check the health of your disc by following the steps in this article.

Performance Test

Performance speeds may vary depending on the make and model of your device. The speed of reading and writing data will be higher, especially if you are using a new type of disk such as SSD. You can even animate your old computer with an SSD.

You can use a variety of software to test the reading and writing performance of your storage device. The most used software in this process is CrystalDiskMark.

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