Most laptop hard drives spin their platters at 4,200 revolutions per minute,
with some later models advancing to 5,400, 7,200, and 9,600 rpm — matching
the speed of larger units designed for desktop computers.The read/write heads in a hard drive are designed to fly on a cushion of air —
much like the wing of an airplane — just above or just below the surface of
the platter. I’m talking about a separation of just a few microns (a micron is a
millionth of a meter, or about^1 ⁄25,000of an inch) from the platter.Most hard drives place all of the heads on a single stalk that moves in and
out. If a single platter is in the laptop drive, the stalk holds two heads — one
for each data surface; if there are two platters, the stalk holds four heads, and
so on. You can see within a modern hard drive, a sacrificial lamb from my
workbench, in Figure 7-1.So, when you put together the speed of the rotating disk and the closeness
of the read/write heads, you can understand how the tiniest speck of dirt or
sand could sit like a boulder on the surface of the platter. That is the reason
hard drives are sealed units; they are assembled in high-tech clean rooms
and protected by ultra-fine filters that allow them to breathe and get rid of
heat but keep dirt away.Figure 7-1:
Inside a
modern
hard drive,
its internal
platters
exposed.118 Part III: Laying Hands on the Major Parts