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 just 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.

• Performing an unconditional deletion of the wrong file. If you hold down the Shift key in Windows when you delete a file, it does not go into the Recycle Bin. You must use third-party data-recovery software to retrieve the file, or eventually it will be overwritten.

• 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 (increasingly common for laptop users), 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 computerswhen you send data from one city to another, for exampleby 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, even more so if the backup medium is low-capacity and cannot hold much data. 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, with the exception of external hard drives, tape backups are among the few backup devices capable of recording the contents of today's multigigabyte drives to a single cartridge.