A mirrored volume is a hard drive or other form of storage media that stores an exact copy of the data from another volume. It is used for fault tolerance, which means mirrored volume serves as a backup device in case the primary device fails.
Companies and organizations that store critical information typically "mirror" the data on another drive. Using a mirrored volume decreases the chance of losing data exponentially. For example, if the odds of a hard drive going bad is 1 out of 200, the odds of two drives going bad at the same time is 1 out of 40,000.
Mirroring data can be done using various backup software programs or may be enabled within operating systems that support disk mirroring. While using a mirrored volume helps keep data safe, it also slows down data transfers because the data must always be written twice. It also costs more, since mirroring data requires twice as much disk storage than what would otherwise be required. However, the cost of an extra hard drive is usually well worth the security of having important data backed up.
Mirroring may also be referred to as "duplexing" or RAID 1.