The Mechanical Hard Disk Drive (HDD) Revolution

A hard disk is part of a unit, often called a “disk drive,” “hard drive,” or “hard disk drive,” that stores and provides relatively quick access to large amounts of data on an electromagnetically charged surface or set of surfaces. A hard disk drive (HDD), hard disk, hard drive or fixed disk is a data storage device used for storing and retrieving digital information using one or more rigid (“hard”) rapidly rotating disks (platters) coated with magnetic material. Hard disk drives (also called hard drives or disk drives) is the mechanism that reads and writes data on a hard disk.

There are several different types of hard drives for storing software and data files: hard disk drives, solid-state drives and external drives. Hard disk drives were introduced in 1956 as data storage for an IBM real-time transaction processing computer and were developed for use with general-purpose mainframe and minicomputers.

Mechanical Hard Disk Drive


The definition of hard disk drive

  • Part computer, part tabulator, in 1956, IBM’s RAMAC was the first machine with a hard disk, which was extraordinary technology at that time.
  • Today’s computers typically come with a hard disk that contains several billion bytes (gigabytes) of storage.
  • The platters are paired with magnetic heads arranged on a moving actuator arm, which read and write data to the platter surfaces.
  • Hard disk drives (HDDs) for PCs generally have seek times of about 12 milliseconds or less.
  • Learn about how they work and how to select the right drive for a particular computer system in this lesson.

The first IBM drive, the 350 RAMAC, was approximately the size of two refrigerators and stored five million six-bit characters (3.75 megabytes) on a stack of 50 disks. Each of its 50 platters two feet in diameter held a whopping 100,000 characters, the total equivalent to five megabytes today. (Images courtesy of IBM.) A fraction of the RAMAC’s size, Seagate introduced the first hard disk with 5.25″ platters in 1979.) Hard disk drives are being used now in gaming systems to provide the best possible experience to enthusiast gamers without forcing them to go for an SSD. A hard disk is really a set of stacked “disks,” each of which, like phonograph records, has data recorded electromagnetically in concentric circles or “tracks” on the disk. Data is accessed in a random-access manner, meaning that individual blocks of data can be stored or retrieved in any order and not only sequentially. Many disk drives improve their performance through a technique called caching.

The innards of hard drives

You may have experienced this scenario: you turn on your computer and you get a black screen with the message ‘Imminent Hard Disk Failure.’ Or, you’ve dropped your laptop on the floor, your screen goes black, and your computer won’t turn back on again. The IBM 350 RAMAC disk storage unit was superseded by the IBM 1301 disk storage unit, which consisted of 50 platters, each about 1/8-inch thick and 24 inches in diameter. Today’s platters are 3.5″ for desktops and 2.5″ for laptops. A “head” (something like a phonograph arm but in a relatively fixed position) records (writes) or reads the information on the tracks. HDDs are a type of non-volatile memory, retaining stored data even when powered off.

Desktop Hard Drives are still widely used

Going mechanical

There are several interface standards for passing data between a hard disk and a computer. More often than not the problem is your hard drive. Whereas the IBM 350 used two read/write heads, pneumatically actuated and moving through two dimensions, the 1301 was one of the first disk storage units to use an array of heads, one per platter, moving as a single unit. See ST506. (Image courtesy of Seagate Technology, Inc.) Now hard disk drives are being internally integrated into game consoles like PS4 offering them large capacity of storage to keep the massive games saved without having to be disposed. The size of a postage stamp, Toshiba introduced a 0.85″ hard drive for mobile devices and shipped 2GB and 4GB units in 2005 and 8GB in 2007. Two heads, one on each side of a disk, read or write the data as the disk spins.

The mechanism of storing data

Introduced by IBM in 1956, HDDs became the dominant secondary storage device for general-purpose computers by the early 1960s. The most common are IDE and SCSI. If your hard drive has crashed, you may have lost all your documents, photos, music, etc. Cylinder-mode read/write operations were supported, while the heads flew about 250 micro-inches above the platter surface. However, solid state USB drives have long surpassed 8GB. (Image courtesy of Toshiba Corporation.) Much less than entry level these days, but in 1998, this Seagate drive’s 47GB was impressive.

There have been lots of reports published on hard disk drives and their bitter rival: Solid state drives, like those released by Anandtech.  Each read or write operation requires that data be located, which is an operation called a “seek.” (Data already in a disk cache, however, will be located more quickly.)A hard disk/drive unit comes with a set rotation speed varying from 4500 to 7200 rpm. Continuously improved, HDDs have maintained this position into the modern era of servers and personal computers. Hard disk drives are sometimes called Winchester drives, Winchester being the name of one of the first popular hard disk drive technologies developed by IBMin 1973. It can be one of the more frustrating experiences for any computer user. Motion of the head array depended upon a binary adder system of hydraulic actuators which assured repeatable positioning.

Various types of hard disk drives

Four decades of development after the RAMAC let us store 100,000 times as much data on the same surface. Disk access time is measured in milliseconds. More than 200 companies have produced HDD units, though most current units are manufactured by Seagate, Toshiba and Western Digital. The primary computer storage medium, which is made of one or more aluminum or glass platters, coated with a ferromagnetic material. Read on to learn how your hard drive works, and some simple steps you can take to avoid losing all your files if it crashes.
The 1301 cabinet was about the size of three home refrigerators placed side by side, storing the equivalent of about 21 million eight-bit bytes. All this is one platter today. (Image courtesy of Seagate Technology, Inc.) Solid-state drives are a relatively new alternative to more traditional hard disk drives. Although the physical location can be identified with cylinder, track, and sector locations, these are actually mapped to a logical block address (LBA) that works with the larger address range on today’s hard disks. As of 2015, HDD production (exabytes per year) and areal density are growing, although unit shipments are declining. Although the terms “hard disk” and “hard drive” are used synonymously; technically, the disk spins inside the drive.

Maxtor hard drive is an obsolete hard drive brand

The brief history of hard drives

  • The hard drive of a computer is a device that stores all the software installed on a computer, as well as all the data files created and used by this software.
  • Access time was about 200 milliseconds.
  • Solid-state drives do not have moving parts, and data is stored electrically instead of magnetically.
  • A hard disk drive (sometimes abbreviated as Hard drive, HD, or HDD) is a non-volatile memory hardware device that permanently stores and retrieves information.
  • The primary characteristics of an HDD are its capacity and performance.

All computers have an internal hard disk for storage, and hard disks in external cases can be plugged into a USB, FireWire or eSATA port for additional storage. This includes any documents you have created and downloaded, such as photos and music. In 1962, IBM introduced the model 1311 disk drive, which was about the size of a washing machine and stored two million characters on a removable disk pack. Most solid-state drives use flash memory, which is also used in memory cards for digital cameras and USB flash drives. There are many variations, but their sizes are generally 3.5″ and 2.5″ for desktop and laptop computers respectively.

The first use of internal hard drives

Capacity is specified in unit prefixes corresponding to powers of 1000: a 1-terabyte (TB) drive has a capacity of 1,000 gigabytes (GB; where 1 gigabyte = 1 billion bytes). Typically, some of an HDD’s capacity is unavailable to the user because it is used by the file system and the computer operating system, and possibly inbuilt redundancy for error correction and recovery. Slowly but surely however, hard disks are being replaced by non-mechanical drives (see solid state drive). Today’s hard disks are “fixed,” which means their platters reside permanently in the drive. The hard drive is a form of permanent storage, rather than temporary memory such as random-access memory (RAM). This means that when you turn off the computer the files remain safely stored on the drive so you can use them again the next time you start your computer. Users could buy additional packs and interchange them as needed, much like reels of magnetic tape. Since there are no moving parts, solid-state drives are much less vulnerable to damage from physical shock.

You can watch the following short video if you need more explanation on what a hard drive really is:

A hard drive consists of one or more platters to which data is written using a magnetic head, all inside of an air-sealed casing. Performance is specified by the time required to move the heads to a track or cylinder (average access time) plus the time it takes for the desired sector to move under the head (average latency, which is a function of the physical rotational speed in revolutions per minute), and finally the speed at which the data is transmitted (data rate). The two most common form factors for modern HDDs are 3.5-inch, for desktop computers, and 2.5-inch, primarily for laptops. In the past, removable cartridges were used for backup and transport (see removable disk). Hard disks are not the computer’s main memory. There are two general types of hard drives: hard disk drives (HDD), which use one or more rotating discs and rely on magnetic storage, and solid-state drives (SDD), which have no moving mechanical parts, but use flash memory like the kind found in USB flash drives. Later models of removable pack drives, from IBM and others, became the norm in most computer installations and reached capacities of 300 megabytes by the early 1980s.

Western Digital releases hard drives for archive purposes

The major downside of solid-state drives is that they are a lot more expensive than hard disk drives, although prices are gradually coming down. Internal hard disks reside in a drive bay, connect to the motherboard using an ATA, SCSI, or SATA cable, and are powered by a connection to the PSU (power supply unit).A hard drive can be used to store just about any type of data, including pictures, music, videos, and text documents. HDDs are connected to systems by standard interface cables such as PATA (Parallel ATA), SATA (Serial ATA), USB or SAS (Serial attached SCSI) cables. Disks store programs and data until deliberately deleted by the user, but memory is a temporary workspace. If you have a regular desktop computer, you most likely have a hard disk drive.

Small capacity yet very high price

Non-removable HDDs were called “fixed disk” drives. Despite the cost, solid-state drives are quickly becoming the preferred type of hard drive for certain types of computers because they are very damage proof and smaller than regular hard disk drives. Computers have a hard drive and use it to store files for the operating system and software that run on the computer, as well as files created or downloaded to the computer by a user. As of 2016, the primary competing technology for secondary storage is flash memory in the form of solid-state drives (SSDs), which have higher data transfer rates, better reliability, and significantly lower latency and access times, but HDDs remain the dominant medium for secondary storage due to advantages in price per bit. To learn how this workspace is used to process data, see memory.
Solid-state drives are more typical for high-end, expensive laptops. Data sent to and read from the hard drive is interpreted by the disk controller, which tells the hard drive what to do and how to move the components within the drive. For example, the MacBook Air now comes standard with a solid-state drive using flash memory. However, SSDs are replacing HDDs where speed, power consumption and durability are more important considerations. For a summary of memory and storage types, see storage vs. memory.

Continuity of development

Every computer has at least one internal hard drive to store software and data. When the operating system needs to read or write information, it examines the hard drive’s File Allocation Table (FAT) to determine file location and available write areas. Although most hard drives are internal, there are also stand-alone devices called external hard drives, which can backup data on computers and expand the available disk space. Hybrid drive products have been available since 2007. Hard disks rotate constantly from 4,000 to 15,000 RPM; however, to preserve battery or power, they can be configured by the user to turn off after a defined period of inactivity.

How does a hard drive work

  • If you are using a Windows operating system, this drive is typically called the C drive.
  • Once they have been determined, the disk controller instructs the actuator to move the read/write arm and align the read/write head.
  • External drives are often stored in an enclosure that helps protect the drive and allows it to interface with the computer, usually over USB or eSATA.
  • These are a combination of HDD and SSD technology in a single device, also known by the initialism SSHD.
  • Capacity is measured in bytes, and the largest drives have passed the terabyte threshold.
  • If you are using a Mac, it is just called the hard drive.
  • Because files are often scattered throughout the platter, the head needs to move to different locations to access all information.

A great example of an external backup device that supports multiple hard drives is the Drobo. Speed is measured by how long it takes to begin transferring data; approximately three to 15 milliseconds (by comparison, CDs/DVDs take 80 to 120 ms) and the rate of transfer is measured in hundreds of megabytes per second. If you need extra storage capacity, you can install additional internal hard drives or connect separate external hard drives.
Some high-performance HDDs were manufactured with one head per track (e.g. Adaptec laptop hard disk drive enclosureExternal hard drives come in many shapes and sizes. See hard drive capacity, access time and transfer rate. Hard disk drives have been the dominant type of storage since the early days of computers. IBM 2305) so that no time was lost physically moving the heads to a track.

There are various infamous hard drive manufacturers that release a small amount of disks to the markets

The role of rotational platters

Some are large, about the size of a book, while others are about the size of a cell phone. Hard disks are pre-formatted at the factory, which divides the platters into identifiable sectors. A hard disk drive consists of a rigid disc made with non-magnetic material, which is coated with a thin layer of magnetic material. Known as fixed-head or head-per-track disk drives they were very expensive and are no longer in production. External hard drives can be very useful since they usually offer more space than a jump drive and are still portable.

The Emergence of External Hard Disk Drives

For more details on disk structure, see magnetic disk, format program, hard disk defect management and drop protection. Data is stored by magnetizing this thin film. In 1973, IBM introduced a new type of HDD codenamed “Winchester”. The picture to the right is an example of a laptop hard disk drive enclosure from Adaptec. Over the years, several kinds of hard disks have been employed.
Now that external hard drives are used for PS4 works to save games and movies, the disk spins at a high speed and a magnetic head mounted on a moving arm is used to read and write data. Its primary distinguishing feature was that the disk heads were not withdrawn completely from the stack of disk platters when the drive was powered down. The user may install a laptop hard drive of any storage capacity into the enclosure and connect it via USB port to the computer. Today, SATA drives are the most common, although SAS drives are also used. A typical hard disk drive operates at a speed of 7,200 rpm (rotations per minute), so you will often see this number as part of the technical specifications of a computer.

Using an External SSD instead

Instead, the heads were allowed to “land” on a special area of the disk surface upon spin-down, “taking off” again when the disk was later powered on. Solid State Drives (SSDs) have started to replace hard disk drives (HDDs) because of the distinct performance advantages they have over HDD, including faster access times and lower latency. For more details, see SATA, SAS, SCSI and hard disk interfaces. The spinning of the disk is also the source of the humming noise of a computer, although most modern hard disk drives are fairly quiet. This greatly reduced the cost of the head actuator mechanism, but precluded removing just the disks from the drive as was done with the disk packs of the day.
While SSDs is becoming more and more popular, HDDs continue to be used in many desktop computers largely due to the value per dollar that HDDs offer over SSDs. In general, hard disk drives are very robust and can be used for many years without problems. Instead, the first models of “Winchester technology” drives featured a removable disk module, which included both the disk pack and the head assembly, leaving the actuator motor in the drive upon removal. However, more and more laptops are beginning to utilize SSD over HDD, helping to improve the reliability and stability of laptops. However, hard disk drives can fail and one of the most common reasons is a head crash.

External Uses of a hard drive

Later “Winchester” drives abandoned the removable media concept and returned to non-removable platters. The first hard drive was introduced to the market by IBM on September 13, 1956. This occurs when the magnetic head scratches the magnetic film. Like the first removable pack drive, the first “Winchester” drives used platters 14 inches (360 mm) in diameter. The hard drive was first used in the RAMAC 305 system, with a storage capacity of 5 MB and a cost of about $50,000 ($10,000 per megavbyte). The hard drive was built-in to the computer and was not removable.
This typically happens as a result of a physical shock, like dropping a computer while it’s on. A few years later, designers were exploring the possibility that physically smaller platters might offer advantages. When your hard drives experience mechanical failure you can often hear a grinding or scratching sound. Drives with non-removable eight-inch platters appeared, and then drives that used a 5 1⁄4 in (130 mm) form factor (a mounting width equivalent to that used by contemporary floppy disk drives). The latter were primarily intended for the then-fledgling personal computer (PC) market. Such a crash results in data loss since the magnetic film gets damaged.


As the 1980s began, HDDs were a rare and very expensive additional feature in PCs, but by the late 1980s their cost had been reduced to the point where they were standard on all but the cheapest computers. It is, therefore, always a good idea to have a backup copy of the important files on your hard drive.

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