There is provided a method and apparatus for implementing a virtual mirror of a primary storage device (106) on a secondary storage device (108). The method comprises providing a map (124a) for translating primary data storage locations on said primary storage device (106) to secondary data storage locations on said secondary storage device (106) and utilizing said map (124a) to enable data stored on said secondary storage device (108) to mirror data stored on said primary storage device (106). By providing such a method, the requirements of the primary and secondary disks (106, 108) can be decoupled such that a smaller secondary disk (108) could be used with a larger primary (106) which will not be filled to capacity. This reduces the unused capacity on the secondary disk (108) which would otherwise be wasted. Further, this approach allows for the replacement of the primary and secondary disks (106, 108) to be independent of each other, provided the size of the secondary disk (108) remains at least as large as the size of the data on the primary disk (106).