Provided are a system, computer program, and method for restoring redundancy in a storage group when a storage device in the storage group fails. In response to detecting a failure of a first storage device in a storage group, wherein the storage group stores each of a plurality of extents in the first storage device and a second storage device to provide redundancy, a determination is made whether a spare storage device that has a storage capacity less than that of the storage group. One of the extents in a storage location in the second storage device that is beyond an upper limit of positions in the spare storage device is moved to a new storage location. The spare drive is incorporated into the storage group to provide redundant storage for the storage group, wherein the extents in the storage group are copied to the spare drive.