What is a Hard Drive?
A hard disk drive (HDD) is a storage device that contains one or more platters that store data, typically using magnetic recording. The platters rotate at high speeds while read/write heads float just above them, accessing data as the platters spin (https://en.wikipedia.org/wiki/Hard_disk_drive).
The main components of a hard drive are:
- Platters – The circular disks that actually store the data. Platters are coated with a magnetic recording material.
- Spindle – The shaft that rotates the platters.
- Read/write heads – The devices that read and write data onto the platters.
- Actuator arm – The arm that holds the read/write heads and moves them across the platters.
Hard drives store and retrieve digital data by magnetizing tiny sections of the platters in patterns corresponding to the binary digits 0 and 1. The platters spin rapidly while the read/write heads float just above them, detecting and inducing these magnetic patterns (https://www.crucial.com/articles/pc-builders/what-is-a-hard-drive). This allows the drive to store large amounts of data and rapidly access it.
History of Hard Drives
Hard disk drives were first invented by IBM in 1956 for use in the RAMAC 305 computer system. The first hard drive was called the IBM 350 disk storage unit and contained fifty 24-inch platters, storing a total of 5 million characters (around 5MB) of data. It was extremely large, weighing over a ton, but innovative for its time.
Over the next few decades, hard drives continued to rapidly evolve. Drives shrank in physical size while exponentially increasing in storage capacity. The IBM 3340 introduced sealed disks and removable disk packs in 1973. The 1970s and 80s saw the introduction of various drive standards by companies like Shugart Associates and Seagate. Winchesters, named after an early model, became a common hard drive design consisting of disks sealed in a rigid flat pack enclosure with the read/write heads flying on an air bearing over the disk surfaces.
In the 1990s, hard drives started to become a standard component in personal computers, replacing earlier floppy and tape drive storage solutions. In 1991, IBM released the 0663 Corsair with an unprecedented 1GB storage capacity. Drive capacities continued growing in the 2000s, enabling new consumer applications and uses. Today, hard drives with multiple terabytes of data are commonplace. While other technologies like solid state drives are emerging, the hard disk drive remains a widely used data storage solution over 60 years since its invention.
How Data is Stored
Data is stored on a hard drive platter’s magnetic surface in concentric circles called tracks. Each track is divided into smaller arcs called sectors, which are the smallest physical storage units on a hard drive. Sectors typically store 512 bytes of data each. Multiple sectors are combined into clusters, which is the smallest unit of storage that can be read or written to by the operating system.
As the read/write head moves over the platter, it can magnetize a tiny spot on the track positive or negative, to represent a binary 1 or 0. By magnetizing many spots in a row, data can be written to the platter. The closer together the tracks and sectors are, the more densely packed the data can be stored, allowing for greater storage capacity. Modern hard drives use advanced signal processing to pack tracks and sectors very closely together.
To write data, the disk controller receives instructions from the host computer to store a file or block of data. The controller determines where the data will be physically located on the disk, moving the heads to that track and waiting for the data to pass under the write head. Then the head magnetizes spots on the track to match the 1s and 0s making up the binary data, thus writing it to the platter. This process happens in parallel if there are multiple platters and heads in the hard drive.
To read data, the disk controller receives instructions for which data to retrieve. The head simply detects whether each spot passing under it is magnetized positive or negative, generating electrical signals that translate to 1s and 0s, thus reconstructing the original binary data. The data is then sent back to the host computer.
Read/Write Heads
Read/write heads are a critical component in a hard drive that enable it to read and write data on the disk platters. As the name suggests, read/write heads perform two key functions – reading data from the platter surface and writing data to it.
The read/write heads are attached to a moving actuator arm that positions them precisely over the platters. Modern hard drives usually have one read/write head per platter surface. So a hard drive with two platters will typically have four read/write heads – two on the top and two on the bottom.
The purpose of the read/write heads is to convert magnetic information on the platters into electrical signals that the drive electronics can understand and vice versa. To read data, the heads detect and convert the magnetic polarity transitions on the platter surface into electrical pulses. The drive electronics then decode these pulses to reconstruct the original data.
To write data, the electronics module sends electrical pulses to the heads that induce magnetic polarity transitions on the platter surface, representing 1s and 0s. The heads have a small electromagnetic coil that generates a local magnetic field to magnetize tiny regions on the platter surface as it flies over it.
The read/write heads float nanometers above the platter surface, almost touching it. They move in unison with the spinning disk allowing them to access data anywhere on the platters (Wikipedia, 2022). The heads are designed to be extremely precise and durable to withstand constant motion over years of use.
Platters
The rigid disks inside a hard drive are called platters. Platters are made of non-magnetic material, usually glass, ceramic, or aluminum alloy coated with a thin layer of magnetic material (Wikipedia). The coating, which stores the data, is normally a thin film of sputtered chrome cobalt alloy or similar material.
Data is stored on both sides of the platter. The number of platters in a hard drive depends on the storage capacity. For example, a 1 terabyte hard drive will typically have two 500 GB platters. More platters allow for greater storage capacity. High performance drives aimed at servers might have as many as 8 or more platters (eBay).
Platters rotate at high speeds while read/write heads float on a thin cushion of air nanometers above the surface to access the data without contacting the platter. The data is written to and read from tracks etched into the magnetic coating.
Single vs. Multiple Disk Drives
Hard drives can have a single physical disk (called a platter) or multiple disks inside the drive enclosure. Both options have advantages and disadvantages to consider.
Single disk drives, as the name suggests, contain just one platter. The entire storage capacity is contained on that single disk. Some benefits of a single disk design include:
- Lower cost – only one platter is needed
- Less noise – no additional moving parts
- Lower power usage
- Increased portability
However, single disk drives also have some downsides:
- Limited capacity – most single disk drives max out at around 2TB
- Slower performance – all the read/write heads are sharing a single disk
- Lack of redundancy – one disk failure results in total data loss
Conversely, drives with multiple disks (platters) inside can provide much greater storage capacity and performance. Some benefits of multi-disk drives include:
- Higher capacity – multiple disks allow for greater total storage space
- Faster speeds – multiple platters allow simultaneous read/write operations
- Built-in redundancy – failure of one disk does not result in total failure
The trade-off is that multi-disk drives tend to be more expensive, physically larger, use more power, and generate more noise and vibration due to the additional moving parts.
In summary, single disk drives provide a compact, quiet, affordable option, while multi-disk drives offer greater performance and capacity for more demanding storage needs.
Common Hard Drive Sizes
The most common hard drive sizes found in desktop computers range from 1TB to 3TB. Many desktop PC hard drives start at 1TB (1,000GB) of storage space, and can be expanded to 2TB, 3TB, 4TB or more. For example, Best Buy lists common desktop PC hard drive sizes as 1TB, 2TB, and 3TB. Higher capacity hard drives are also available, like 4TB, 6TB, 8TB, 10TB or more, which are well suited for users with large storage needs.
For laptops, the most common hard drive sizes tend to be smaller, typically starting at 250-500GB. Many mainstream laptop models include either a 256GB or 512GB hard drive. Some higher end gaming or multimedia laptops may go up to 1TB. According to TechTarget, typical laptop hard drive sizes include:
- 250 GB
- 320 GB
- 500 GB
- 750 GB
- 1 TB
So in summary, desktop PCs commonly have hard drives in the 1-4TB range, while laptops tend to have smaller drives from 250GB to 1TB.
How Many Platters and Disks
The number of platters and disks in a typical hard drive varies depending on the storage capacity. Smaller drives usually have just one or two platters, while larger drives can have up to seven or more platters [1].
For common hard drive sizes:
- 500GB to 1TB hard drives often have two to three platters [1].
- 2TB hard drives usually have three to five platters [2].
- 3TB to 4TB hard drives tend to have five to seven platters [2].
Each platter has two disk surfaces that can store data. So a hard drive with two platters would have four disks total. With three platters there are six disks, and so on.
In general, higher capacity hard drives have more platters and disks to provide the increased storage space. But the total number can vary between models and brands.
Recent Trends
In recent years, hard drive technology has continued to evolve with new form factors emerging as alternatives to traditional HDDs. Solid state drives (SSDs), which use flash memory instead of spinning platters, have become increasingly popular due to their faster speeds, lower power consumption, and improved reliability compared to HDDs (Coughlin, 2023). While more expensive per gigabyte, SSD prices have dropped steadily, allowing them to displace HDDs in many consumer devices. However, HDDs still maintain an advantage in high capacity, networked storage applications.
Hybrid hard drives, or SSHDs, combine HDD capacity with SSD speed by including a small amount of high-speed flash memory to cache frequently accessed data. Although SSHDs didn’t gain widespread adoption, they represented an attempt to blend the strengths of SSDs and HDDs (Mordor Intelligence, 2023).
New form factors like 2.5″ and M.2 SSDs have enabled the design of thinner and lighter laptops and devices. On the HDD side, smaller 2.5″ drives are now commonly used in laptops and external portable drives. Overall, while HDDs still dominate in the data center, SSD adoption continues to grow across desktops, laptops, and consumer devices.
The Future of Hard Drives
Hard disk drives have come a long way in terms of capacity and performance, but there are predictions that they will continue to evolve in the coming years. According to Secure Data Recovery, new technologies like OptiNAND are emerging, which combine hard disk drives with embedded flash drives to optimize performance [1]. Guardians Data Destruction also discusses how new hard disk drive storage technologies are evolving to compete with SSD and continue meeting the needs of data centers [2].
In terms of capacity, The Future of Storage by Data Barracks suggests hard drives could reach up to 100TB by 2025 through new technologies [3]. Compared to today’s largest hard drives of around 20TB, this would be a huge leap forward. Speeds are also predicted to increase with new methods like microwave-assisted magnetic recording allowing data to be written more quickly. Form factors may shift as well, with greater use of smaller 2.5″ drives rather than larger 3.5″ drives in many applications. Overall, experts predict substantial advances in HDD technology in the coming years when it comes to capacity, performance, physical design and more.