Hard Disk Drives (HDD)

A hard disk drive (HDD) is a non-volatile storage device which stores digitally encoded data on rapidly rotating platters with magnetic surfaces. HDDs were originally developed for use with general purpose computers.

Over time the need for large-scale, reliable storage, independent of a particular device, led to the introduction of dedicated systems such as RAID arrays, Network Attached Storage (NAS) and Storage Area Networks (SAN) that provide efficient and reliable access to large volumes of data for a variety of applications and vertical markets. Further to this HDD usage has expanded into consumer applications such as home entertainment systems & storage, camcorders, mobile phones, digital audio players, digital video players, digital video recorders, personal digital assistants, back up devices and video game consoles.

HDDs record data by magnetizing material directionally, to represent either a 0 or a 1 binary digit. They read the data back by detecting the magnetization of the material. A typical HDD design consists of a spindle which holds one or more flat circular disks called platters, onto which the data is recorded. The platters are made from a non-magnetic material, usually aluminum alloy or glass, and are coated with a thin layer of magnetic material.

The platters are spun at very high speeds from 5400 to 15,000 rpm. Information is written to a platter as it rotates past devices called read and write heads that operate very close (tens of nanometers in new drives) over the magnetic surface. The read and write head is used to detect and modify the magnetization of the material immediately under it. There is one head for each magnetic platter surface on the spindle, mounted on a common arm. An actuator arm (or access arm) moves the heads on an arc across the platters as they spin, allowing each head to access almost the entire surface of the platter as it spins.

Today’s heads write using perpendicular recording (Perpendicular Magnetic Recording, PMR), the benefit of perpendicular recording over the traditional technology of longitudinal recording, writing data across the platter, is that it can deliver three times the amount of storage density. Current drives using longitudinal recording have an estimated limit of 200Gbit per square inch whereas perpendicular recording offers around 1Tbit per square inch (1000Gbit per square inch).

The advantage of perpendicular recording is that it achieves higher storage densities by aligning the poles of the magnetic elements, which represent bits, perpendicularly to the surface of the disk platter, as shown in the illustration. Aligning the bits in this manner takes less space on the platter than longitudinal recording so data blocks can be placed closer together, thus increasing the number of magnetic elements that can be stored in a given area.

Longitudinal & Perpendicular Recording