Brief description of Hard Disk Drives (HDD's)

Brief description of Hard Disk Drives (HDD's)

A HDD is a very complex electromechanical device which employs many technologies.
It is comprised of the HDA (Head Disk Assembly) and the PCB (Printed Circuit Board). For the purpose of this manual, we will concentrate on the HDA.

The HDA consists of a spindle motor, disks (the media) , Read/Write heads, an actuator to move the head assembly (Head Stack) to the target data block, all contained within a sealed enclosure. The data is written onto rotating disks which are magnetically treated. The rotational speed for Server HDD's ranges from 5400-RPM to 10000-RPM with the majority of drives used today being at 7200-RPM.

While the disk rotates, the Read/Write heads fly above the disk. The fly height is typically 1.8 to 2 micro inches (and getting lower as technology progresses). As a comparison, a human hair is 3000 micro inches (a micro inch is one millionth of an inch). The actuator moves the head stack assembly (up to 20 heads may be installed in a head stack assembly) onto the desired location (track) to write or read the data.
The disk is segmented radially into tracks and each track is made up of sectors.
The sector is the least addressable unit on the disk drive - this is where the data reside.
A sector is 512 bytes long. So a 4.5GB drive will contain 8,789,062 sectors.
In reality, disk drives have many more sectors than the stated capacity.

This is because perfect media (the disks) are not possible with today's technology. Thus, in the manufacturing process, some sectors are found to be unusable (deficient magnetic coating, pits, etc.), and are reassigned to spare sectors somewhere else on the drive. The drive will have the advertised capacity when leaving the factory as well as throughout the life of the drive.

The amount of data that can be stored on a disk is a measure of it's areal density - defined as the number of data bits stored on the disk per square inch. Thus a typical 4.5GB drives has an areal density of 0.8-Gbits to 1-Gb per square inch. In the next 5 years drives will reach areal densities of greater than 5-Gb/sq. inch (IBM Almaden research has already demonstrated 5Gb/sq inch in a laboratory environment). In order to achieve these densities, improvements in head and media technologies have to occur.
The advent of MR (Magneto Resistive) and GMR (Giant MR) heads as well as improvements in media have paced the advances in areal densities. In order to understand why media defects can occur a brief overview of media manufacturing is presented in 'Media Manufacturing'.

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