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What is HDD Storage? The 7 Types you need to know

By Tibor Moes / January 2023

What is HDD Storage? The 7 Types you need to know

HDD Storage

Many of us spend years clicking the save button without giving a second thought about where the saved data goes. This, of course, changes the moment something goes wrong with the hard drive.

If that happened to you, you’ve probably found yourself deep in the rabbit hole of HDD research. Luckily, we have all of the relevant information about this storage type in this article. Read on to learn everything concerning HDDs, including how they work, and which types exist currently.

Summary: Hard disk drive (HDD) storage is a device that stores your data permanently. Unlike other storage types, an HDD in proper working condition will never lose the information on it, even when left without power.

Tip: Don’t take risks online. Protect your HDD against malware with antivirus software and safeguard your online privacy with a VPN.

What is HDD Storage?

A hard disk is defined as a non-volatile storage device. Being non-volatile means that the data storage device will maintain everything stored on it regardless of whether it has power or not.

Other examples of non-volatile storage would be cloud storage (which is just disk storage accessed through online connections) or a USB.

Desktop computers need some kind of data storage device to function properly. The HDD is the most common type used today, although some variants are becoming increasingly popular. However, hard disk drives aren’t necessary only for computers. Other devices like consumer electronics, data centers, and mobile devices use HDDs, too.

What Are Hard Disk Drives Made Of?

The main elements of an HDD are the hard disk itself, spindle, actuator, logic board, and the case.

You’ll notice that the first component mentioned is the hard disk, which might cause some confusion. The term “hard disk” is used interchangeably with “hard disk drive” in everyday language. Technically speaking, though, “hard disk drive” refers to the entire device, while “hard disk” means the actual disk platter.

With that technicality out of the way, let’s look at how each component contributes to the proper function of a hard disk drive.

Hard Disk

All stored data resides in the hard disk. In all traditional hard disk drives, disk platters are made of glass or aluminum and covered with special layers that enable data writing and provide protection.

Hard disks are, as the name implies, shaped like round disks. Their layers include:

  • The core
  • Magnetic layer
  • Protective layer

The Core

As mentioned, the core of a hard disk is usually made of aluminum. However, modern manufacturers have started to use glass, ceramic, or a composite of the two. The reason for the change has to do with the purpose of the hard disk core.

The core is meant to do only one thing within the disk: provide sturdy support for the magnetic layer. That’s why the core must be rigid and stable, but those two traits aren’t the only ones that matter.

The material making up the core also needs to be light and resistant to higher temperatures. In addition, it should be as flat and smooth as possible.

The aluminum alloy that was used until relatively recently performed well enough in most regards. Its widespread use was also due to its availability and affordable price. Yet, as the hard disk drive technology progressed, the traditional material started to fall behind.

Modern hard disk drives are pushing the boundaries of storage capacity and speed. This means greater density of stored data and faster rotation, which increases the drive’s working temperature. As a result, hard disks now need to be thinner yet more robust.

New materials, particularly the glass-ceramic composite, have all those traits. Disks made with a glass-ceramic core can endure higher temperatures without getting damaged or cracked. At the same time, their surfaces are smoother, allowing for the actuator arm to slide closer to the hard disk.

Magnetic Layer

The magnetic or media layer is where data is stored. This layer is made of a cobalt alloy or iron oxide and is microscopically thin. Even today, manufacturers are experimenting with other materials to maximize the density of stored data without jeopardizing reliability.

The magnetic layer is applied to both sides of the hard disk. This is usually done either by electroplating or vacuum deposition.

Electroplating is the same process used in certain jewelry. On the other hand, vacuum deposition is a complex technique that would be too technical for this article. We must mention, though, that this method has an amazing alternative name: magnetron sputtering. Add “ray” at the end, and the title would be worthy of a comic book villain’s doomsday device.

Protective Layer

Since the magnetic layer is extremely thin and, by extension, very delicate, a protective layer is added on top of it. This layer is by no means thick, but it provides protection from accidental contact with the actuator, as well as from harmful dust particles.

Although the protective layer can keep the magnetic surface safe, it can’t prevent damage caused by more substantial impact. If such damage happens, data recovery from the disk might even be impossible.

The Spindle

Hard disk platters are essentially spinning disks that need to rotate constantly when data on them is written or read. The spindle provides that rotation.

The spindle assembly consists of a central rod connected to a motor. The platters are firmly attached to the rod to ensure maximal stability. If there are multiple platters, each must be secured so that it spins at the exact same speed as the rest.

The motor represents a tiny mechanical wonder. Spindle motors are crafted to provide the same performance level for as long as the hard disk drive is in use. In other words, the spinning platters will maintain a consistent number of rotations per minute (RPM) for years.

On the subject of rotations per minute, hard disk drives come with spindles of different speeds. Based on that parameter, they can come in 4K, 7K, 10K, and 15K variants. The actual speeds for each type are:

  • 4K: 4,200 RPM
  • 7K: 7,200 RPM
  • 10K: 10,000 RPM
  • 15K: 15,000 RPM

The Actuator

The actuator is responsible for the read/write function. It’s an assembly consisting of three crucial parts:

  • Actuator motor
  • Actuator arm
  • The read/write head

The motor is responsible for arm movement across the hard disk. This mechanism functions on the principle of spring-loaded coils and magnets. Like its spindle counterpart, the actuator motor is a marvel of technology. It provides rapid, extremely precise movement, placing the arm and read/write head at the exact position in milliseconds.

Actuator arms are directly attached to the motor. They are designed to move across hard disk platters in arcs so that each part of the writable area is covered. In hard disk drives with multiple platters, each disk is assigned one arm and all arms move in the same direction and speed.

Actuator arms need to move with pinpoint accuracy and can’t have even the slightest vertical oscillation, i.e., up or down wiggling. This is because each arm carries the read/write head that is suspended very close to the platter. In fact, the distance between the platter and the read/write head is only large enough to fit about 40 atoms.

The Read/Write Head

The read/write function in hard disk drives might sound relatively simple, especially in association with how we read and write. However, this process is quite complex, as is the read/write head itself.

The head is an electromagnetic device built at a microscopic scale. It’s about 100 nanometers wide and 10 nanometers thick. As the head hovers above the platter, it uses electrical current to detect or change the magnetic polarity of individual bits on the platter’s magnetic layer. Depending on whether the head needs to read or write data, it will communicate with the logic board and interact with the platter nearly instantaneously.

Although it’s a tiny component, the head requires advanced technology and plenty of time to produce.

Read/write heads are made of aluminum titanium carbide, a type of ceramic. The entire production process for a single head includes about 200 steps and can last up to two months.

What’s even more fascinating is that the head consists of two parts, one for reading and the other for writing. The reading element determines changes in the magnetic field while the writing part emits a magnetic field of its own. The trick here is that the writer needs to generate a sufficiently strong field to shift the magnetic polarity of platter bits. This power is achieved with one or several micro-coils.

The Logic Board

The logic board is responsible for proper operation of every hard disk component. It controls both spindle and actuator motors, as well as the data being read or written to the platters.

This central hard disk drive unit features the necessary circuitry and two crucial chips.

The first is a read-only memory (ROM) component that contains the hard disk firmware. This firmware is used for operation management.

The second is a random access memory (RAM) chip which creates a memory buffer that can boost input and output performance.

The Case

The hard disk casing is designed to isolate other components from external impurities, provide a stable foundation for the moving parts, and minimize noise and vibration.

Since even the tiniest specs of dust could damage the disk or get in the way of the reading head, the case must be airtight. The exception to this are some hard drive models with ventilation openings. However, these openings are always filtered and the case is otherwise completely sealed.

There are also hard drive cases filled with helium instead of air. Helium has lower density, temperature, and weight than air which means less friction. As a result, all components function more efficiently and the storage device reduces its power consumption.

A helium hard drive should be able to sport thinner platters than the regular one and possibly allow for greater storage density.

Hard disk cases should be opened only in controlled environments unless the disk is being destroyed or recycled.

How Hard Disk Drives Work

As storage devices, hard disk drives are designed to store data from systems such as personal computers. Data storage functions without constant power consumption, although the hard drive will, naturally, need to be powered when users want to access data on it.

The data storage process begins with communication between the hard drive and software. In devices like desktop computers, the software in question is usually the operating system or a computer program.

Operating systems store user data, configuration files, and system files on hard disks. Similarly, applications may use the data storage device to write configuration, settings, application, and user files.

On the other hand, software can issue a command to retrieve data previously stored on the hard drive.

Regardless of the direction of the communication – whether it’s for input or output – the logical and mechanical components of the hard drive will function much the same.

Suppose the operating system issues a command to create a new file. The command would go to the hard drive logic board. From there, the board would communicate with the actuator assembly, giving it specific instructions.

The assembly motor would then move the arm across the platter to get the head in the right position. Finally, the head would write data by changing the polarity from positive to negative, which equates to 0 and 1 in the binary system.

Hard Drive Data Organization

The data on a hard drive is organized into tracks, sectors, and clusters. This organization makes the data easily accessible, enabling the device to write and read at greater speeds.

Tracks represents thin concentric circles on the platter. These are packed tightly with each track containing hundreds of thousands or even several million bytes of information. Platter tracks are also numbered: the outermost is track zero and the numbers gradually increase moving inwards.

Every track is divided further into sectors, which represent the main data storage units on a hard drive. Each sector has a number of bytes reserved for error detection, control, and potential corrections. Beside those reserved bytes, sectors can hold upwards of 512 bytes of data. A track can contain several thousand sectors.

Finally, clusters represent groupings of sectors. This grouping exists to protect the data from being overwritten. If you haven’t heard of clusters, you probably know about a phenomenon linked with them: hard disk fragmentation.

Hard disk fragmentation happens when a cluster isn’t made of physically adjacent sectors. For instance, you might have a cluster consisting of four sectors but there’s no available space to write data right next to the first sector. In that case, the rest of the data will be written on the nearest available sectors.

When data is written in this non-contiguous manner, the hard disk is considered fragmented. Since the head needs to travel more to read fragmented data, this results in slower performance. Hard drives become most efficient when filled with sequential data. In this case, those would be contiguous sectors. Additionally, fragments are not the same size. Over time, fragmentation leaves empty spaces that programs can’t use effectively.

HDD Types

Hard disk drives are most often separated into groups according to how they connect to the motherboard. Hard drives in general can also be classified by their principle of operation. However, not all hard drive types in this classification contain a disk, so they’re technically not considered HDDs.

Furthermore, there are differences in file system structures, usage, and HDD form factors. Yet, neither of those aspects classifies hard drives into a particular type.

HDD Types by Connection

Historically, there have been many advancements in connection technology for storage devices. Further progress is happening even today, leading to several different connection types existing at the same time in the modern market.

Types of hard disk drives by connection include:

  • SATA
  • SCSI
  • SAS


You’ve likely noticed that only the first type has three names. This is due to the relatively complicated history of the IDE connection.

IDE is an acronym for Integrated Drive Electronics. This was an early standard for storage drive connection which kept evolving both in form and name throughout the decades.

The first time this standard changed its name was when it became fully standardized. The famed computer manufacturer, IBM, integrated the standard into one of their flagship products – the Advanced Technology (AT) computer. As a result, the connection type was standardized as the Advanced Technology Attachment or ATA.

For a long time, ATA was the go-to solution for local storage connections. Naturally, hard disk drives produced during that period were designed to use this technology. Since ATA became the foundation for further development, it didn’t come as a surprise when a superior successor appeared.

That successor was the SATA connection, which will be covered later on.

When SATA came out, ATA went through its second renaming to differentiate one standard from the other. It became the Parallel ATA or PATA.

A PATA connection contains 40 or 80 wires with corresponding connection pins. A modern desktop computer might feature a PATA port, or the connection can be established via an adapter. In such cases, the 40-wire variant might hinder write speeds that the rest of the computer is capable of and the 80-wire option might provide better performance.


Although SATA is the newer variant of the IDE/ATA/PATA model, the connection technology first saw light of day nearly two decades ago. The “S” in the name stands for Serial.

Yet, like its predecessor, SATA was subject to several improvements, with the latest significant upgrade coming out in 2009. Today, SATA is still a respectable format. Unlike IDE/ATA/PATA, this connection type doesn’t suffer from slower data transfer rates and is still supported by the latest operating systems.

Various modern devices use SATA, from computers and servers to gaming consoles like the PS4. SATA hard disk drives have become renowned for its dependability, prolonging their popularity and market longevity.

A SATA hard drive will usually last for years if free from manufacturing defects. Once its performance starts to falter, it likely won’t suffer a catastrophic failure. Instead, it will succumb to wear and tear gradually, making sudden data corruption less likely.

SATA uses only seven conductors, which is a great improvement over the dozens used by previous IDE legacy technology. On top of that, SATA hard drives usually hold more data and have superior speed. SATA connectors have also remained a standard in computer manufacturing, making the format by far the most versatile.


The SCSI interface was invented in the 1980s, not long after IDE. This standard isn’t a part of the IDE/ATA family but represents an independent development.

SCSI stands for Small Computer System Interface. Unlike other acronyms on the list, SCSI wasn’t blessed with a vowel in the middle so the tech world had to make a unique pronunciation for it. If you hear someone talking about SCSI hard drives, they’ll be referred to as “Scuzzy.”

Initially, SCSI was designed for use in Unix and Mackintosh computers, but its use soon expanded to PCs as well. For some time, this parallel connection type remained highly regarded, receiving its latest update in 1996.

At the time, the standard was widely praised for its speed and reliability, as well as lower CPU usage. That last feature was due to a dedicated controller that handled most of the data traffic to and from SCSI hard drives.

But Scuzzy didn’t enjoy its fame for much longer after that.

As the SATA system kept evolving, it managed to surpass SCSI in various aspects. Most notably, SATA hard drives were considerably faster. As a result, while you can still find machines utilizing SCSI, that format has largely fallen out of use.

Although SCSI didn’t have success in keeping up with increasing requirements, it gave birth to a new connection format that proved to be quite an upgrade – SAS.


Serial Attached SCSI or SAS made a significant change compared to the standard it originated from. The difference is right in the name: SAS is a serial connection type instead of the parallel one employed by SCSI.

Serial communication in SAS uses point-to-point links for data transfer, allowing an SAS hard drive to store data at superior speeds.

The increased performance of SAS made it more than a worthy successor to SCSI. Even better, the new connection type was based on the same command set that SCSI used. This meant that SAS hard drives were backwards-compatible with SCSI-based devices.

That compatibility serves as an advantage for SAS even today.

This connection type has seen plenty of development. Unlike the formats we’ve covered so far, advancements in SAS technology are continuing today. SAS received its last update as recently as 2017 when SAS-4 came out. This version of the technology can transfer data at an impressive 22.5 gigabits per second (Gbps). SAS-5 is currently in the works, and its expected speeds should reach 45 Gbps.

Types of Hard Drives by Operation Principle

In terms of operation principle, hard drives can be disk drives, solid state drives, and hybrid. By now, you already know that hard disk drives utilize spinning disks or platters. In contrast, solid state drives (SSD) function more akin to a flash memory. Finally, hybrid hard drives use both principles.

Since the traditional hard disk drive is the very subject of this article, we won’t retread what was already covered. Still, this might be a good opportunity to mention some HDD-specific points of concern. In particular, we’ll break down the aforementioned file system structures and form factors.

Hard Disk Drive – File System Structure and Form Factors

A file system structure represent the way in which a hard disk drive is formatted. The formatting will depend on the operating system. Mac and Linux operating systems are relatively straightforward in this regard since each has a single format: HFS for Mac and EXT for Linux.

These formats have slight variants. For instance, HFS also has HFS+ and EXT can appear as EXT2, 3, or 4. But those variants don’t change the formatting drastically.

The same can’t be said about hard disk formatting in Windows. Here, users are faced with three options:

  • FAT
  • NTFS

FAT stand for “File Allocation Table” and can come in three variants: FAT12, FAT16, and FAT32. However, the first two are intended only for smaller drives. FAT 32 supports the largest file sizes of the three – up to 4 gigabytes – and can handle partitions up to 32 gigabytes.

It’s obvious that even FAT32 as the best option has severe limitations. For that reason, this format is falling out of use more and more. Instead of FAT32, most users opt for the NTFS system.

NTFS allows for much larger files, allowing users to take advantage of their full HDD capacity. Retrieving data from an NTFS drive is just as straightforward as with FAT, but there are no limitations. Still, this formatting system has some drawbacks, too.

Firstly, NTFS is developed exclusively for Windows machines. This means that the file system won’t be usable if a different OS is installed. In other words, if you replace Windows with Linux, you won’t be able to use NTFS. In the worst case, data recovery could be challenging.

Secondly, NTFS is a poor fit for solid state drives or any other flash-based memory device. This system will work optimally with HDDs but might shorten the lifespan of other devices.

Finally, exFAT represents the opposite to NTFS. This file system is explicitly designed for SD cards, USBs, and SSDs. The issues with exFAT are that, like the NTFS, it’s geared towards Windows, and might not be supported by older devices.

Form Factors

Form factors in HDDs represent the physical size of the drive. The idea of form factors was introduced ever since the floppy drive was invented. Initially, there were many variants in sizes and drives could come in the following form factors, expressed in inches:

  • 0.85
  • 1
  • 1.3
  • 1.8
  • 2.5
  • 3.5
  • 5.25 FH
  • 5.25 HH
  • 8

Of all those sizes, most became obsolete, partially to allow for the widespread use of an universal drive bay.

Today, there are only two common form factors: 2.5 and 3.5. The former are usually used in laptops, while the later are reserved mostly for desktop computers.

Solid State Drive (SSD)

A solid state drive doesn’t feature any moving parts. Instead, it stores data on a series of interconnected flash memory chips. This technology has numerous advantages, including faster data recovery, smaller physical size, impressive storage capacity, and greater data integrity.

At the same time, solid state drives share the same benefit that hard disk drives have – they don’t need a connection with a power supply unit to maintain the stored data.

In terms of connection, SSDs mostly use SATA, although they have the upper hand compared to other types. Namely, a solid state drive can mount directly onto the computer motherboard, provided it has the appropriate slot.

With the improvements in technology, SSDs have become a serious contender in the race between storage devices. Today, they represent the preferred option, making most people think that the technology is relatively new. However, actual facts are quite different.

The History of the SSD

If you were told that the SSD is older than the HDD, you’d likely have difficulties believing the statement. Yet, that’s precisely the case.

The earliest ancestor technologies of SSD started all the way back in the 1950s. Of course, there was no mention of computer chips at that time, but storage options for the vacuum tube computers of the day were somewhat similar to how SSDs function now.

The technology that likely planted the seed of solid state drives was the magnetic core memory. This was a type of RAM which used magnetic rings for data storage. Unlike the later-developed HDDs, though, core memory didn’t require an active moving mechanism.

While it was revolutionary, this MIT invention fell out of use with the advent of another technological marvel: the semiconductor. Yet, the idea behind core memory stuck. Thanks to that, it didn’t take long before the first SSDs that resemble modern storage devices appeared on the market.

The year was 1976, and the industry giant Dataram introduced their Bulk Core drive. It boasted an awe-inspiring storage capacity of 2GB. Despite still being based on the magnetic core technology, this device provided a fascinating look into the future.

Development continued throughout the 1980s until the following decade saw the first flash memory SSDs. The market immediately recognized them as superior to HDDs, except for one aspect: the price.

When they showed up, flash-based SSDs could reach more than $40,000 in price. Naturally, this wasn’t a viable solution for the wider market of private computer users.

Luckily for everyone, the answer to the pricing issue came relatively soon, in the first half of the 2000s. First, Transcend broke new ground with their PATA SSDs that could be bought for as low as $50. Then, during the same decade, the devices kept growing in storage capacity while remaining relatively affordable.

Today, SSD storage space can measure in terabytes and most users can afford them.

What’s Inside an SSD?

As previously mentioned, the SSD is vastly different than an HDD. Rather than containing primarily mechanical parts, an SSD consists of chips and circuitry. The common components include:

  • Circuit board
  • Flash memory
  • Cache storage
  • Controller chip
  • Connection interface

The circuit board, of course, houses all other components and provides the operating environment. Integrated circuits transport data between the cache and flash memory, while the controller takes care of the read/write function.

When it comes to interface, that aspect can make a significant difference between solid state drives.

Solid state drive Types by Interface

SSDs can have various interface types, including:

  • SATA
  • M.2 SATA
  • mSATA
  • M.2 PCI-E
  • M.2 NVMe

SATA connections have already been explained thoroughly. Those used with SSDs are the same type found in many internal HDDs. Other types, though, will require some explanation.

All M.2 connections plug directly into the motherboard and require no cables. It’s worth mentioning that not all computers will feature the appropriate M.2 slot.

M.2 SATA represents a smaller and more convenient version of SATA. Besides being more compact, there’s not a lot that differentiates the two SATA variants. For instance, a SATA and M.2 SATA SSD of the same capacity will also have the same transfer speeds.

Similar could be said about the mSATA. However, these types of SSD are even more compact and lightweight. They could represent a better solution for laptops due to their decreased power consumption which may prolong battery life.

M.2 PCI-E drives plug into the PCI-E slot which boasts greater speeds than most connections – the “E” stands for “express.”

Finally, NVMe means “Non-Volatile Memory express.” In other words, it’s a slot specifically designed to accommodate data storage devices. Of course, this type of SSDs has better performance than others due to its specific intended use. However, it does come at a higher price and might reduce battery life due to its power drain.

Hybrid Drives

Solid state hybrid drives (SSHD) represent a middle ground between SSDs and HDDs. They feature the same mechanism as HDDs, as well as flash memory used as a data cache.

Unfortunately, this middle ground is far from an ideal solution. Hybrid drives might be, on average, physically smaller than HDDs, but they’re still larger than the solid state variant. Yet, they lack the speed of an SSD.

The only thing that goes in favor of an SSHD is the pricing. This type of drive is less expensive than others.


Comparing SSD vs. HDD drives, it’s easy to conclude that the solid state type excels on most accounts. While HDD speed is nothing to scoff at, SSDs surpass them in most cases, especially when plugged into designated express slots.

In terms of storage space, SSDs can be quite large, although HDDs are still leading the race, albeit only slightly. The largest hard disk internal drives reach 20 terabytes, with some external HDDs going even beyond. SSDs aren’t far behind – internal SSDs can get to 16 terabytes. Some of those options can compare to cloud storage capacity.

However, it’s worth noting that SSDs are still quite pricey, especially when you get to the higher ends of storage space. That’s why hard disk drives will likely remain the winner when capacity is considered.

Continuing with SSD vs. HDD, we should mention external drives of both types. While the capacity is comparable, SSDs have the advantage of being smaller and, by extension, more convenient. Still, this doesn’t mean HDD external drives are demonstrably worse in all cases.

The final piece of the puzzle comes in place when we look at pricing. Here, again, HDDs come on top in the SSD vs. HDD challenge. There was plenty of talk about SSDs getting more affordable, and their prices have certainly gone down in recent years.

But an equally sized SSD is still far more expensive than its HDD counterpart. And chances are that trend won’t change anytime soon, or at least until the hard disk drive goes out of use.

Making an SSD vs. hybrid comparison wouldn’t have much point. The only advantage that hybrids have over SSDs is the pricing, but it usually comes with a sharp drop in quality.



Frequently Asked Questions

What does HDD storage mean?

HDD, or hard disk drive, is memory storage that uses a spinning magnetic disk to store information. The data stored on HDD storage can theoretically be kept indefinitely without a power source if the device is in perfect conditions.

What's HDD storage good for?

This storage type can be used for various purposes, from personal computers to data backup and more. You can also find a hard disk drive in gaming devices like the PlayStation and Xbox.

Is HDD the same as a hard drive?

The two terms are used interchangeably, but don’t mean the same thing. A HDD is the entire device, while the hard drive is an internal component of that device. People sometimes also use the term hard drive to refer to long-term PC storage as a whole, which includes SSDs.

Author: Tibor Moes

Author: Tibor Moes

Founder & Chief Editor at SoftwareLab

Tibor is a Dutch engineer and entrepreneur. He has tested security software since 2014.

Over the years, he has tested most of the best antivirus software for Windows, Mac, Android, and iOS, as well as many VPN providers.

He uses Norton to protect his devices, CyberGhost for his privacy, and Dashlane for his passwords.

This website is hosted on a Digital Ocean server via Cloudways and is built with DIVI on WordPress.

You can find him on LinkedIn or contact him here.

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