He asked Wozniak to develop another method of secondary storage. Galaksija used cassette tape as secondary storage. Some drum memories were also used as secondary storage. Memory is usually classified by access rate into primary storage and secondary storage. To date, data deduplication has predominantly been used with secondary storage systems. This usually occurs when the node data are in secondary storage such as disk drives. "Tertiary storage" or "tertiary memory" is a level below secondary storage. Non-volatile memory is typically used for the task of secondary storage, or long-term persistent storage. Here the active processor copies the status of system periodically into secondary storage. Operating systems have memory areas that are "pinned" (never swapped to secondary storage). Some systems allow partitions to be "swapped out" to secondary storage to free additional memory. Thus, secondary storage is significantly slower than primary storage. Typical secondary storage devices are hard disk drives and solid-state drives. Unlike modern computers, magnetic tape was also often used for secondary storage. An extra 20-bit word per tube stored an offset value into the secondary storage.