Hard drive basics | Capacities, RPM speeds, interfaces and mechanics

hard drive basics
Welcome to our guide to the hard drive

In this guide we aim to give you an idea of just what a hard drive is. We cover what these ubiquitous devices are and their uses in today's world.

We then go on to explain hard drive capacities, RPM speeds and the interfaces used to connect them. We finish with a look at the internal components of the hard drive.
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What is a hard drive and what does it do?

A hard drive is a mass storage device found in all PCs (with some exclusions) that is used to store data such as the operating system, installed software and a user's files.

The data on hard drives can be erased and/or overwritten. The hard drive is classed as a non-volatile storage device, which means it doesn't require a constant power supply in order to retain the information stored on it (unlike RAM).

Inside every hard drive are round disk-like objects called platters which are made of either an aluminium/alloy or a glass/ceramic composite. Each platter is coated with a special magnetic coating enabling them to store data magnetically.

Hovering above these platters are read/write heads that transfer data to and from the platters. We cover platters, heads and the other mechanical elements in more detail below.

Hard Drive Capacities

Hard drives come with various different storage capacities which are measured in bytes. Common capacities are stated in MB (Megabytes) and GB (Gigabytes).

There is, understandably, some confusion among consumers regarding the storage capacity of hard drives. This is due to the fact that most hard drive manufacturers and many software applications associate the terms MB (megabyte), GB (gigabyte) and TB (terabyte) with different values.

Hard drive manufacturers tend to use the decimal interpretation (SI prefix) and lots of software packages use the binary interpretation (binary prefix). This means that 1GB = 1000MB according to the manufacturer but the software's file system classes 1GB as 1,024MB.

Below shows the difference between the decimal and binary prefixes.

PREFIX DECIMAL VALUE BINARY VALUE
MB 1,000,000 bytes (10002, 106) 1,048,576 bytes (10242, 220)
GB 1,000,000,000 bytes (109) 1,073,741,824 bytes (10243, 230)
TB 1,000,000,000,000 bytes (1012) 1,099,511,627,776 bytes (10244, 240)

As you can see there is a large disparity between the values, especially as storage sizes get larger. For example, if you purchased a hard drive stating it had 300GB you may find that your software reports the storage capacity as only 279GB.

To avoid such confusion the terms MiB, GiB and TiB are used to describe the binary interpretations, unfortunately the terms have not seen widespread use.

In recent times there has been a surge in hard drive technology, allowing massive storage capacities which would have seemed impossible not many years ago.

Common hard drive capacities these days range from 120GB up to and exceeding 1TB.

To put that in perspective, you can store around 33,333 text documents (around 30,000 bytes each) per 1GB of hard drive space, or around 238 MP3 music files (around 4,194,304 bytes each) for each 1GB. When you consider a hard drive with a storage capacity of 500GB, it is clear the capacity of modern hard drives is massive.

Hard Drive RPM Speeds

You will often see hard drives advertised as being capable of a certain RPM (Revolutions Per Minute). This figure (as the name suggests) refers to how many times the spindle makes a complete 360° turn in any single minute.

The higher the RPM, the faster the data can be read from the platters, which increases overall performance. RPM values range from about 5,400RPM to 12,000RPM and above.

Hard Drive Interfaces

Hard drives are available with a number of different interfaces, these interfaces provide connectivity and facilitate communication between the hard drive and the rest of the system.
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The various interfaces each have their own specifications.
  • IDE (Integrated Drive Electronics)
    • Although superseeded by the SATA interface due to SATA's improved performance, IDE hard drives are still used in some systems.
  • USB (Universal Serial Bus)
    • Used to connect external USB devices. Although the actual interface on the hard drive is usually SATA, there is a USB-SATA bridge which facilitates the communication.
  • SATA (Serial Advanced Technology Attachment)
    • The SATA interface has become the most commonly used interface in modern desktop PCs and laptops. It offers much faster and more efficient data transfer than its predecessor IDE (PATA).
  • SCSI (Small Computer System Interface)
    • The SCSI interface was, and still is, very popular with power-hungry users. Although most desktop PC and laptop manufacturers incorporate SATA interfaces into their systems, SCSI devices and interface cards are still available. Recent adaptations which allow the connection of SCSI devices include Serial Attached SCSI (SAS) and USB Attached SCSI (UAS). SCSI is still used in performance-critical devices like servers.

Hard drive mechanics

Hard Drive Mechanics
The mechanical technology used inside hard drives hasn't changed that much over the years, in fact the basic mechanics in today's hard drives are much the same as they were 10 or 15 years ago.

In fig 1.1 to the left you can see the internal mechanics of a hard drive:
  • A - Platter/s
  • B - Read/Write Head/s (and slider)
  • C - Actuator Arm/s
  • D - Actuator
  • E - Spindle

Platters (A in fig 1.1)
Most Platters are made of either an aluminium/alloy or glass/ceramic composite and are like glass to the touch. There is always more than one platter inside modern hard drives and each platter is double sided (with some exceptions).

In modern hard drives the platters spin at 5400RPM and above. The distance (flying height) between the read/write heads and the platter is around 50nm (0.05µ, a human hair is around 100µ).

You should never open your hard drive's sealed casing (unless you are in a controlled environment such as a clean room) as a single speck of dust is bigger than the gap between the platters and read/write heads. If dust/debris does enter the drive, it can have catastrophic results when the heads crash into it.

The platters have a magnetic coating which allows them to store the data magnetically.
Read/Write Head/s (B in fig 1.1)
These are attached to the end of each actuator arm and as the name suggests they are responsible for reading and writing data to and from the platters. There is usually a set of read/write heads on each side of each platter although some drive configurations have an odd number of heads.
Actuator Arm/s (C in fig 1.1)
Actuator arms move across the platters to position the read/write heads in the right place to read or write the required data. The actuator arms are manufactured so they have a spring action causing them to close if there is no platter present, this ensures they do not move away from the platter with time. As the drive winds up to full speed the air that is generated by the spinning motion lifts the sliders (and therefore the heads) around 50nm (0.05µ) above the platters.
Actuator (D in fig 1.1)
This refers to the device that physically moves the actuator arms. Years ago they used to use stepper motors for controlling the actuator arms, but the problem with stepper motors in applications such as this is that over time and with a lot of use, they lose their integrity and can cause data corruption.

Another problem with stepper motors is that when they get hot (which hard drives do) they lose their precision. For example, data that is saved with a cool drive can sometimes be unavailable when the motor gets hot due to this displacement effect.

Stepper motors also required the read/write heads to be parked (moved to a data-free area of the drive). If the read/write heads were not parked and the drive suffered any shock (such as transportation), the data could be corrupted.

Nowadays most (if not all) hard drives use a voice coil instead of a stepper motor, unlike stepper motors, voice coils are linear and don't suffer from the same integrity problems.

Because voice coils are magnetically driven by electrical currents there is no mechanical wear and tear. Voice coils also negate the need for parking the heads before shutting off the power, as when they lose power the heads return to the parked position automatically.
Spindle (E in fig 1.1)
The spindle revolves the platters, when you see an RPM (Revolutions Per Minute) specification of a hard drive it is referring to the spindle speed.
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