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A hard drive, also known as a hard disk, is a common form of mass storage for computer data. The drive consists of several disks, called platters, arranged in a stack. They are made of rigid, durable material that is coated with a ferromagnetic substance similar to that used in audio or video tape. The platters are spaced a fraction of a centimeter apart. Each has two sides (top and bottom) and two read/write heads (one for the top and one for the bottom). The assembly is enclosed in a sealed cabinet. Figure 33-2A is an edgewise, cutaway view of the platters and heads in a typical hard drive.
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Drive Action When the computer is switched off, the hard-drive mechanism locks the heads in a position away from the platters. This prevents damage to the heads and platters if the computer is moved. When the computer is powered up, the platters spin at several thousand revolutions per minute (rpm). The heads hover a few millionths of a centimeter above and below the platter surfaces. When you type a command or click on an icon telling the computer to read or write data, the hard-drive mechanism goes through a series of rapid, complex, and precise movements. The head positions itself over the particular spot on the platter where the data is located or is to be written; then the head detects the magnetic fields and translates them into tiny electric currents. All this takes place in a small fraction of a second. Data Arrangement and Capacity The data on a hard drive is arranged in concentric, circular tracks. There are hundreds or even thousands of tracks per radial centimeter of the platter surface. Each circular track is broken into a number of arcs called sectors. A cylinder is the set of equal-radius tracks on all the platters in the drive. Tracks and sectors are set up on the hard drive during the initial formatting process. There are also data units called clusters. These are units consisting of one to several sectors, depending on the arrangement of data on the platters. Figure 33-2B is a face-on view of a single hard-disk platter, showing a track and one of its constituent sectors.
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33-2 At A, an edgewise view of a hard drive, showing four platters.
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At B, a broadside view of a platter, showing one track and one sector.
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When you buy a computer, whether it is a desktop, notebook (also called laptop), or portable (also called handheld) unit, it will have a hard drive built in. The drive comes installed and formatted. Most new computers are sold with several commonly used programs installed on the hard drive. Some computer users prefer to buy new computers with only the operating system, by means of which the programs run, installed; this frees up hard-drive space and gives the user control over which programs to install (or not to install).
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There are several types of external storage (besides the hard drive) in which data can be kept in large quantities. Computer experts categorize external storage in two ways: access time and cost per megabyte (or gigabyte, or terabyte).
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Disk Media Disk media offer the advantage of speed, convenience, and reliability. For personal computers, there are many forms of external disks. Here are three of the most well-known. An external hard drive is exactly what its name suggests. This type of device is exceptionally fast, and has storage capacity similar to the hard drives inside computers. They can be easily connected to any personal computer with a short cable. Most of these devices require a power supply, often found in the form of a brick that contains a transformer, rectifier, and filter that converts 120 V ac into the necessary dc for the device. Compact disk recordable (CD-R) and compact disk rewritable (CD-RW ) are popular for backing up and archiving computer data. You can buy these disks for various other applications, too, such as storage for digital photos and home videos. They are the same size, physically, as conventional CD-ROMs, which are used for commercial software, databases, and digital publications. The main asset of CD-R and CD-RW is moderately large capacity and long shelf life. Most new computers have built-in drives for these disks. Diskettes, also called (imprecisely) floppies, are about 9 cm (3.5 in) in diameter and enclosed in a rigid, square case about 4 mm (0.15 in) thick. Their capacity, individually, is limited. They are all but obsolete. Increasingly, new computers are sold without drives for floppies. Tape Media The earliest computers used magnetic tape to store data. This is still done in some systems. You can get a tape drive for making an emergency backup of the data on your hard drive, or for archiving data you rarely need to use. Magnetic tape has high storage capacity. There are microcassettes that can hold more than 1 GB of data; standard cassettes can hold many gigabytes. But tapes are extremely slow because, unlike their disk-shaped counterparts, they are a serial-access storage medium. This means that the data bits are written in a string, one after another, along the entire length of the tape. The drive might have to mechanically rewind or fast-forward through a football field s length of tape to get to a particular data bit, whereas on a disk medium, the read/write head never has to travel farther than the diameter of the disk to reach a given data bit. Flash Memory Flash memory is an all-electronic form of storage that is useful especially in high-level graphics, bigbusiness applications, and scientific work. The capacity is comparable to that of a small hard drive, but there are no moving parts. Because there are no mechanical components, flash memory is faster than any other mass-storage scheme, provided it does not cause a software conflict with other programs in the computer. (Software conflicts can cause a computer to slow down or freeze up. ) Flash memory is available in small modules roughly the size of your index finger, and can be plugged directly into one of the Universal Serial Bus (USB) ports provided in all new computers. Some flash memory modules come in the form of PC cards (also called PCMCIA cards), which are credit-card-sized, removable components.
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