If all your backups are in one place, then they are subject to a single-point failure such as the following:

Theft: Burglars steal your computers and all your backup tapes too.

Fire: The entire building burns down destroying all your computers and all your backups.

Flood: The entire building is washed away destroying all your computers and making your backups very soggy.

Authorities: The tax office mixes up your address with that of the dodgy computer importer next door. In a sudden raid, they confiscate all your computers and backup tapes. After the mixup is resolved you go to the pound to try to retrieve your computers, but discover that they have been mislaid.

In addition to these external threats, onsite backups are also vulnerable to a range of organizational threats:

Co-Worker: A co-worker working late at night needs a tape in a hurry, and grabs your backup tape because "you won't notice". The next day when your disk fails, you do notice.

Wrong tape: Someone puts the wrong tape in the tape drive.

Wrong label: You put the wrong label on the tape and lose it amongst the other tapes.

Sabotage: A disgruntled employee decides to destroy all your backups.

Cleaners: The cleaners think your backup tape is for the office Christmas party. The next day when you come in, you find your tapes draped around the ceiling tied to balloons.

All these dangers mean that it's essential to ensure that at least some of your backups are physically separated from the rest so as to ensure that a single natural or human disaster cannot wipe out ALL the copies of your data. Such backups are usually referred to as "offsite backups".

The best way to ensure that you perform regular offsite backups is to link the procedure to an existing procedure. For example, if you are in the habit of visiting Aunt Agatha every Sunday, get into the habit of taking a backup tape with you. If you don't have an Aunt Agatha, there are a number of reputable companies that will visit your business and collect a backup from you for storage in a secure, temperature controlled warehouse.

As a last resort, it's a good idea to keep at least one backup in a safe-deposit box in the vault of a large bank.