A mirrored RAID system is essential in a mission-critical environment to ensure continuity of data access. But this doesn’t amount to the same thing as having a fail-safe backup strategy. For that, you need to perform scheduled backups to a secondary set of drives, which should be stored in a safe location such as a fireproof safe or somewhere offsite. In a simple office setup, you could use one external drive to hold the main Lightroom image library, and a duplicate drive of similar capacity to keep a regularly backed-up copy of the data. The important feature of this kind of setup is that backups can be scheduled manually. If a problem were to occur on the master disk, such as an accidental file deletion or a directory corruption, you would have the opportunity to rectify the problem by copying data from the backup drive. If you keep the data on a separate drive, it can be stored in a separate location away from the main computer.
Running scheduled drive backups reduces your chances of losing data. However, as long as files are stored on read/write disk media, they’ll always be vulnerable to accidental erasure or a virus attack that could infiltrate from the masters to the backup drives. To reduce the risk further, make DVD copies of your files and keep those DVDs in an appropriate storage location. Of course, reloading all the data from DVDs would be a pain, but writing data to DVD ensures that the data stays free from virus attack or human error. A DVD is the most economical removable media storage offered at present. But keep an eye out for newer media systems, such as Blu-ray, that are bound to become more affordable and practical for storing larger amounts of data on single disks.