But not only do you need more than one copy, you should also have one of your copies located off site in case of major disaster. Consider this scenario: One of my customers religiously performed backups of their entire network, but chose to keep all copies in their server room. Fire broke out one evening, and their entire archive was either heat-, smoke- or water-damaged, or else it was burnt to a crisp. Their equipment was replaced rather quickly, but their backup media needed to be sent out for recovery. Had they stored their information off site, they: 1) wouldn't have needed a data recovery service, and 2) would have been back up and running shortly after the equipment was installed.
This was first published in September 2004