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).
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.
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 micro cassettes 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 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.