Hard Drive Basics, Hard Drives and their capacities.

hard drive basics
Welcome to our guide to hard drive basics
In this section we aim to give you an idea of just what a hard drive is.

We cover what these ubiquitous device 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.

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 permanent data such as the operating system, programs and user 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 small round disk-like objects made of either an aluminium/alloy or a glass/ceramic composite. These are called platters, 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 will cover platters, heads and the other mechanical elements in more detail in the hard drive mechanics section.

Hard Drive Capacities

Hard drives come with many different storage capacities, hard drive capacity is measured in bytes, with common capacities being 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.

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.

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.

The various interfaces each have their own specifications.
  • IDE (Integrated Drive Electronics)
    • Although superseeded by the SATA interface due to SATA's imrpoved performance, see below, IDE hard drives are still used in some systems.
  • USB (Universal Serial Bus)
    • Used to connect external devices.
  • 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.. SCSI is still used in performance-critical devices like servers.

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