Figure 1-21.—Internal hard disk drive mounted on disk drive chassis

To review, multiply bytes (5 12) times sectors (15) times tracks (80) times sides (2). This gives the total capacity of the diskette, 1.2M bytes. Write Protect Feature.— Like all other media, floppy disks/diskettes must also have a way of being protected from a user accidentally writing on them. This is accomplished with a write-protect notch (or cut-out) located in the upper right-hand corner of the plastic jacket (see figure 1-19). Whether or not the write-protect notch is covered to protect the data on the diskette depends solely on the disk drive manufacturer. For example, on most systems using 5 1/4-inch diameter diskettes, covering the write-protect notch makes it impossible to write on the disk’s surface. However, the opposite is true on systems using 3 1/2- inch diameter diskettes. Uncovering the write-protect notch makes writing impossible. In short, find out which is which on your particular system before you accidentally wipe out an entire diskette’s data. Floppy Disk Drives.— Floppy disk drives are manufactured to read and write data in one of several bit densities for both single- and double-sided diskettes. A single-sided disk drive can only read a single-sided diskette. However, a double-sided disk drive, like the one shown in figure 1-20, can read both a single-sided and a double-sided diskette. H A R D D I S K S A N D H A R D D I S K DRIVES.— Although most microcomputer systems you will be working on use some type of floppy disk drive, you should also be aware that all micros also contain a small sealed unit called a Winchester disk or hard disk drive. Unless the disk drive is an external type unit, you might never know it was there. Floppy disk drives are easy to spot; all you have to look for is the drive’s door, that open slot where you insert the diskette. Hard disk drives, on the other hand, are normally sealed Figure 1-20.—A floppy disk drive being removed from system unit. Figure 1-21.—Internal hard disk drive mounted on disk drive chassis. units that can be tucked away just about anywhere. Generally, you will find them inside the system unit (figure 1-21) in the space occupied by one of the floppy drives. Hard disk drives provide you with many timesaving features not available nor possible with the floppy disk drives. These include increased access speeds, greater storage capacity (up to 8 gigabytes [GB] of storage), and overall convenience. Working with hard disk is much easier because you can quickly end one program and start another, all through the operating system. Hard disk units consist of rigid metal platters that are tiered or stacked. In most cases, the disks themselves are not removable, and for this reason can be hermetically sealed in the storage unit along with the access mechanism that contains the read/write heads. Because this type of disk drive is completely sealed and free from air contamination, the read-write heads can be positioned to within 20 millionths of an inch from the surface of the disk. This also allows the disk to be rotated at a high rate of speed-typically, 3600 revolutions per minute. Hard disks also comes in two sizes (diameters): 5.25 inches, and 3.50 inches, with 3.50 being the most popular. Their storage capacities range from 500 megabytes to 8 gigabytes, with the majority in the 1.2- to 2-gigabyte range. As mentioned earlier, hard disks can be placed internally within the system unit, like the one shown in figure 1-21, or externally as a detachable unit, as shown in figure 1-22. You may have also noticed that the disk drive shown in figure 1-22 is a portable hard drive in that the hard disk is removable. The hard disks and read/write mechanism are enclosed within a rugged case. The result is a self-contained hard disk you can insert like a floppy and remove for reasons of security. 1-20