Hard disks break down for the same reason cars or refrigerators break down:
They're mechanical, and mechanical items are susceptible to physical problems. But
hard disk failures can be more alarming, because they're more mysterious.
First let's examine hard disk construction and operation. A hard disk comes
packaged in an airtight container. This container prevents contaminants such as dirt,
liquid, dust particles, and hair from coming in contact with the disk's platters, which
are sealed inside. The platters themselves are composed of a substrate and a magnetic
medium. The substrate, the platter's base material, is either aluminium or (more
common today) glass or ceramic. The substrate material must be nonmagnetic and
capable of being machined to a smooth finish.
To allow data storage, both sides of each platter are coated with a magnetic
medium, formerly magnetic oxide but now almost exclusively a layer of metal called a
thin-film medium. The thin-film medium stores data in magnetic patterns, with each
platter capable of storing a billion or so bits per square inch (bpsi) of platter surface.
To record and retrieve data, the hard disk assembly contains read/write heads. Each
platter surface (top and bottom) typically has a corresponding R/W head, with each
R/W head attached to a mechanical arm and all mechanical arms attached to a pivot.
With the platters rotating around a spindle and the arms precisely positioning the
heads, the entire structure is reminiscent of the good old record players gathering dust
in many of our basements.
A main difference is that record turntables play only one of the stacked albums
while the rest wait to drop into place, whereas in the hard disk assembly all of the
platters are available at all times, and each has its own arms and R/W heads. Another
difference is that the turntable's arm has no built-in intelligence; it simply holds the
stylus, which is guided by the physical grooves of the album. In a hard disk assembly,
the arm swings across the head according to instructions from a precise motor called
an actuator; the platter itself does not guide the head. Instead, the actuator moves the
R/W heads to the exact physical location of the data requested by the application.
There is yet another difference between record players and hard disks: While
the stylus touches the vinyl album, the R/W heads float a few microinches above the
platter. On early disks, this distance was 10 microinches or more, but today's R/W
heads float at 5 microinches. The heads ride atop the airflow caused by the spinning
of the platters, and except for a couple of circumstances, they never come in contact
with the platters. If they did, they would damage or even destroy the data where they
landed (hence the need for ultrasmooth platters and coatings).
The planned circumstance under which the heads touch the platters occurs when
the disk stops spinning-that is, when the disk is powered off. During the spin-down
period, the airflow diminishes and then stops completely, and the heads are no longer
held off the platters. The unplanned circumstance occurs during a heavy shock of
some sort, when the head cuts through the airflow and makes contact with the platter.
I. Read the text using a dictionary.
II. Make up short stories to answer the questions below. Use the lists of key
words and word combinations.
Generated by Foxit PDF Creator © Foxit Software
http://www.foxitsoftware.com For evaluation only.