Disclosed is a method of collapsing a derivative version of a primary storage volume into the primary storage volume. The method comprises generating the derivative version of the primary storage volume that contains a plurality of data items stored in a secondary storage volume, wherein the derivation version comprises a plurality of blocks, identifying changed blocks of the plurality of blocks that changed as a result of modifying at least one of the data items, identifying which of the changed blocks of the plurality of blocks that changed remain allocated, and collapsing the derivative version of the primary storage volume into the primary storage volume by copying those blocks identified as changed and allocated to the primary storage volume.