IX. Text D. Hard Disk Troubles

Introduction

Be prepared and don't panic: Many errors are not fatal; you just have to know how to deal with them.

By Neil Randall

Of all the problems a PC can have, none is more frightening than a hard disk error. The problem could be as easy to deal with as a few bad sectors or a loose power connector, but whenever we see anything wrong with hard disk operation, our minds conjur up the horrifying phrase "disk crash”.

After all, most of us don't back up our data as often as we should, so a full disk crash could result in permanent losses. Even those who do regular backups will usually have to buy and install a new disk and then figure out the backup software's restore procedure. At the very least, it's a time-consuming and stressful procedure, and Murphy's Law ensures that these things happen only when. deadlines loom and stores are closed.

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.

Text E. The new way of looking at things:

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