Hard Disks

Hard disks store all the data in your computer. First developed in 1956 by IBM, the 305 RAMAC was 16 square feet, weighed over a ton and had to be transported by plane. It had the grand capacity of 5MB of storage space and was leased for $3,200 per month ($54,984 in 2021 dollars). At right you can see the RAMAC hard disk being unloaded from a cargo plane with a forklift.

Today's hard drives are only 3.5" in diameter, weigh around 400 grams, can hold 600,000 times as much data and costs about $100. Below is a typical 2TB disk found in modern computers.

How does a hard drive work? Below is a typical disk drive that has had its cover removed so you can see the inner workings.

Click on each label for an explanation of each part of the drive.

Platters

Platters are the "disk" in the eponymous "disk drive", and are circular disks of aluminium, glass or ceramic, coated with a magnetic material which store the actual data. Most drives have multiple platters to increase the storage capacity of the drive and are rigid or hard (from whence we get the term "hard disk") and polished to a mirror smooth finish. Both sides of the platter can be used for storage, requiring two heads per platter to read/write the data.

Spindle

The spindle is the central axis on which the platters are mounted, with enough space in between the platters to allow the read/write heads to obtain the data stored on them. The spindle is attached to the spindle motor which spins the platters at a constant speed, ranging from 5400 rpm for a standard desktop drive, up to 15,000 rpm for high performance enterprise drives only found in servers.

Read/write heads

The read/write heads are attached to the end of the head arm and fly over the disk on a thin cushion of air, a few nanometres above the surface of the disk platter. The read heads convert the data stored as magnetised sections on the disk into electrical impulses that can be understood by the computer. The write heads reverse this procedure, converting the electrical signals from the computer into magnetic patterns on the disk platters. Magnetic density has been steadily increasing over the years, allowing larger and larger disk storage. Currently 15TB disks are available, using new technologies.

Head arm

The head arm moves the heads over the disk surface. All heads move simultaneously as a single unit, individual heads cannot move to another track. The head arms are lightweight but rigid, allowing them to move quickly to the selected track. The arms are not solid, but triangular constructions to reduce mass. Rapid seek times are the direct result of lighter head arms.

Voice coil actuator

The head arms are moved across the disk surface by the voice coil actuator, so called because the electric coils resemble the ones found in audio loudspeakers. The coils are positioned between powerful magnets and when an electric current is fed through the coil it generates a magnetic field that causes it to move in relation to the stationary magnets. Voice coil actuators move the heads very quickly and very accurately to the required track on the disk, but even with today's technology, the movement of the heads is still a thousand times slower than the electronics in the disk.

Air Filter

Many people think that there is a vacuum inside hard disks, but what would the heads float on? Air is present, free from all impurities as the gap between the head and the platter is smaller than a human hair and any impurity could result in damage to the head or the platter. As air expands and contracts due to temperature and altitude variations, the air filter lets completely purified air into or out of the interior of the drive

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