Since the mid-1980s, the primary storage device used by computers has been the hard disk drive. However, for data backup, data transport between computers, and temporary storage, secondary storage devices such as high-capacity removable media drives, floptical drives, magneto-optical drives, flash memory devices, and tape drives are useful supplements to primary storage.

The options for purchasing removable devices vary. Some removable-media drives use media as small as a quarter or your index finger, whereas others use larger media up to 5 1/4''. Most popular removable-storage drives today have capacities that range from as little as 16MB to as much as 100GB or more.

These drives offer fairly speedy performance and the capability to store anything from a few data files or less frequently used programs to complete hard disk images on a removable disk or tape.

Extra Storage

As operating systems and applications continue to grow in size and features, more and more storage space is needed for these programs as well as for the data they create.

Operating systems aren't the only program types that are growing. Applications whose MS-DOS versions once fit on a few floppy disks have now mutated into "everything but the kitchen sink" do-it-all behemoths that can take 500MB or more of disk space.

The multimedia revolution—fueled by powerful, low-cost digital cameras, scanners, and video recorders—enables you to capture and store images that easily can consume hundreds of megabytes of space, and the MP3 craze is filling countless gigabytes of storage on individual users' systems with digitized musical hits and classics.

High-capacity removable storage devices provide the capability to easily transport huge data files—computer-aided drawing (CAD) files and graphics files, for example—from one computer to another. Or, you can use removable storage to take sensitive data away from your office so you can lock it safely away from prying eyes.

Some types of removable-media storage feature archival durability, whereas others are designed for the "shoot it today, delete it tomorrow" world of digital photography.

Backing Up Your Data

Any computer book worth reading warns repeatedly that you should back up your system regularly. Backups are necessary because at any time a major problem, or even some minor ones, can corrupt the important information and programs stored on your computer's hard disk drive and render this information useless. A wide range of problems can damage the data on your hard drive.

Here is a list of some of these data-damaging problems:

• Sudden fluctuations in the electricity that powers your computer (power spikes), resulting in data damage or corruption.

• Overwriting a file by mistake.

• Mistakenly formatting your hard disk when you meant to format a floppy.

• Hard drive failure resulting in loss of data that has not been backed up. Not only do you have to install a new drive, but, because you have no backup, you also must reinstall all your software.

• Catastrophic damage to your computer (storm, flood, lightning strike, fire, theft, and so on). A single lightning strike near your office or home can destroy the circuitry of your computer, including your hard drive. Theft of your computer, of course, is equally devastating. A recent, complete backup greatly simplifies the process of setting up a replacement computer.

• Loss of valuable data due to a computer-related virus. One single download or floppy disk can contain a virus that can damage valuable files and even your entire hard disk. With several hundred new viruses appearing each month, no antivirus software program can keep you entirely safe. A recent backup of uninfected, critical files can help repair even the worst damage.

Backups are also the cure for such common headaches as a full hard drive and the need to transfer data between computers. By backing up data you rarely use and deleting the original data from your hard drive, you free up space once occupied by that data.

If you later need a particular data file, you can retrieve that file from your backup. You also can more easily share large amounts of data between computers—when you send data from one city to another, for example—by backing up the data to a tape or other media and sending the media.

Regardless of how important regular backups are, many people avoid making them. A major reason for this lapse is that for many people, backing up their systems is tedious work when they have to use floppy disks or other low-capacity media.

When you use these media, you might have to insert and remove many disks to back up all the important programs and data. Optical storage, high-capacity magnetic media, and tape backups are all useful devices for making backups.

Historically, tape backups have been regarded as the most powerful of these technologies because tape backups are among the few backup devices capable of recording the contents of today's multi-gigabyte drives to a single cartridge for restoration.

Disk, Tape, and Flash Memory

Several types of removable-media disk drives are commonly used. Traditionally, the most common varieties have used magnetic media, but some use one of two combinations of magnetic and optical storage: floptical or magneto-optical.

Magnetic media drives use technology similar to that of a floppy or hard disk drive to encode data for storage. Floptical and magneto-optical media drives encode information on disk by using different combinations of laser and magnetic technologies.

Some tape drives are also capable of emulating disk drives by providing drive letter access to a portion of the media but are used primarily to perform streaming backups of large disk drives and network drive arrays.

Magnetic Disk Media

Whether you are looking at "pure" magnetic media, floptical media, or magneto-optical drives, all types of magnetic disk media share similar characteristics. Disk media is more expensive per megabyte or gigabyte than tape, usually has a lower capacity, and is more easily used on a file-by-file basis as compared to tape.

Disk media uses random access, which enables you to find, use, modify, or delete any file or group of files on a disk without disturbing the rest of the disk's contents. Disk media is faster for copying a few files but is typically slower for copying large numbers of files or entire drives.

Magnetic Tape Media

Tape media has much less expensive costs overall per megabyte or gigabyte than disk media, has a higher total capacity, and is more easily used on an image or multiple-file basis. Tape drives use sequential access, meaning that the contents of a tape must be read from the beginning and that individual files must be retrieved in the order found on the tape.

Also, individual files usually can't be modified on the tape or removed from the tape; the contents of the entire cartridge must be deleted and rewritten. Thus, tape drives are more suited for complete backups of entire hard disks including all applications and data. Because it is suited for mass backup, tape can be difficult to use for copying single files.

Flash Memory Media

The newest type of removable storage is not magnetically based but uses flash memory—a special type of solid-state memory chip that requires no power to maintain its contents.

Flash memory cards can easily be moved from digital cameras to notebook or desktop computers and can even be connected directly to photo printers or self-contained display units. Flash memory can be used to store any type of computer data, but its original primary application was digital photography.

However, more and more digital music players have removable flash memory cards, and so-called thumb or keychain flash memory devices that plug directly into a USB port are helping to make flash memory a mainstream storage medium and an increasingly popular replacement for some types of magnetic removable-media storage.