Techniques for migrating data of a logical device from a source to a target system may include: obtaining a map indicating unused data portions of a logical device; issuing read commands from the target system to the source system to only read data from the source copy of the logical device for logical addresses that are not marked in the map as unused data portions; receiving the read data at the target system; writing the read data to second logical addresses of the target copy of the logical device on the target system; and marking logical addresses indicated by the map as corresponding to the unused data portions. The source system may also determine whether all data of a received read request that reads data from the logical device is included in an unused data portion and if so, return no read data and a specialized error condition.