A dispersed storage network (DSN) includes a processing unit and multiple storage unit sets. A first storage unit set and a second storage unit set receive, sets of encoded data slices generated, by the DS processing unit, for storage. The encoded data slices include dispersal-encoded portions of one or more data objects. The first and second storage unit sets are used to mirror storage of the one or more data objects. The first storage unit set stores data slices unsynchronized to the second storage unit set, and generates a record identifying the unsynchronized data slices. Later, the storage unit detects that the availability of at least one storage unit of the second storage unit set, and in response facilitates copying of particular unsynchronized data slices to corresponding storage units of the second storage unit set. After copying, the record is updated to indicate the synchronization.