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In data storage, a disk mirror is a complete separate copy of a data set.
Explanation
Mirroring is a common function found in enterprise storage and RAID devices. In particular, RAID 1 is a disk mirroring scheme.
It is differentiated from a snapshot in that there are no remaining links between the original (or source) and the copy (or mirror).
Other benefits of mirroring
In addition to providing an additional copy of the data for the purpose of redundancy in case of hardware failure, disk mirroring can allow each disk to be accessed separately for reading purposes. This effectively doubles the speed of disk read access (halves the seek time). This is an important consideration in hardware configurations that frequently access the data on the disk.
The mirror disk can also be split off and used for data backup, allowing the first disk to remain active. However merging the two disks then requires a synchronization period.
Other schemes
Some mirroring schemes employ three disks, with two of the disks for the redundancy mirroring and the third to be split off for performing backups. This third disk is sometimes called a "business contingency volume".
"Disk mirror"