What is a Hard drive

What is a hard drive and what does it do?

A hard drive is a mass storage device found in all PCs (with some exclusion) 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).

To understand these figures correctly you need to know the basics of how data is stored /processed in digital systems such as PCs.

Digital data is a series of 1's (ones) and 0's (zeroes) which are referred to as bits

(Binary digITS), a byte is made up of 8 of these bits, so a single byte of data may look like 01001011 (8 consecutive bits).

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 Diagram below 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

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).

note: glass/ceramic platters are a fairly recent introduction, some of the early experiments using glass for platters had to deal with the fact that glass runs (very, very old glass windows will be thicker at the bottom than the top). So theoretically as the glass is being spun around at 5400RPM, it will eventually run out of shape.

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 (more on how the data is stored later).

Read/Write Head/s

These are attached to the end of each actuator arm and as the name suggests they are responsible for reading and writing data to/from the platters. There is usually a set of read/write heads on each side of each platter (it is common to have hard drives with an odd number of heads).

Actuator Arm/s

Actuator arms move across the platters to position the read/write heads in the right place to read/write the data required. The actuator arms are manufactured so they have a spring action causing them to close (if 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 lifts the sliders (and therefore the heads) around 50nm (0.05µ) above the platters.