Hardware Reference
In-Depth Information
continue to store data perfectly to this day with their lids either on or off. Keep in mind, of
course, that these drives don't contain any valuable information.
If you want to see what hard disks look like on the inside without opening them up, I suggest
checking out the Red Hill Hardware Guide's “old gold” hard drive history page at
The Ultimate HDD Analogy
There is an old analogy that compares the interaction of the heads and the medium in a typical HDD
as being similar in scale to a 747 Jumbo Jet flying a few feet off the ground at cruising speed (500+
mph). I have heard this analogy used repeatedly for years, and in the past I even used it myself
without checking to see whether the analogy was technically accurate with respect to modern hard
drives. It isn't.
Perhaps the most inaccurate aspect of the 747 analogy is the use of an airplane of any type to describe
the head-and-platter interaction. This analogy implies that the heads fly low over the surface of the
disk, but technically, this is not true. The heads do not fly at all in the traditional aerodynamic sense;
instead, they float or ski on a cushion of air that the platters drag around.
A much better analogy would use a hovercraft instead of an airplane; the action of a hovercraft much
more closely emulates the action of the heads in an HDD. Like a hovercraft, the drive heads rely
somewhat on the shape of the bottom of the head to capture and control the cushion of air that keeps
them floating over the disk. By nature, the cushion of air on which the heads float forms only in close
proximity to the platter and is often called an
air bearing
by those in the disk drive industry.
I thought it was time to come up with a new analogy that more correctly describes the dimensions and
speeds at which an HDD operates today. I looked up the specifications on a specific HDD and then
magnified and rescaled all the dimensions involved by a factor of more than 300,000. For my
example, I use an IBM Deskstar 75GXP drive, which is a 75GB (formatted capacity), 3 1/2-inch
ATA drive. The head sliders (called
pico
sliders) in this drive are about 0.049 inches long, 0.039
inches wide, and 0.012 inches high. They float on a cushion of air about 15 nanometers (nm or
billionths of a meter) over the surface of the disk while traveling at an average true speed of 53.55
miles per hour (figuring an average track diameter of about 2 1/2 inches). These heads read and write
individual bits spaced only 2.56 micro-inches (millionths of an inch) apart, along tracks separated by
only 35.27 micro-inches. The heads can move from one track to another in 8.5 milliseconds during an
average seek.
To create my analogy, I magnified the scale to make the head floating height equal to 5 millimeters
(about 0.2 inches). Because 5 millimeters is about 333,333 times greater than 15 nanometers (nm), I
scaled up everything else by the same amount.
Magnified to such a scale, the heads in this typical hard disk would be about 1,361 feet long, 1,083
feet wide, and 333 feet high. (The length and height would be about equal to the Sears Tower if it
were tipped over sideways.) These skyscraper-sized heads would float on a cushion of air that to
scale would be only 5mm thick (about 0.2 inches) while traveling at a speed of 17.8 million miles per
hour (4,958 miles per second), all while reading data bits spaced a mere 0.85 inches apart on tracks
separated by only 0.98 feet!
The proportionate forward speed of this imaginary head is difficult to comprehend, so I'll elaborate.
The diameter of the earth at the equator is 7,926 miles, which means a circumference of about 24,900
