The Masstor M860 was delivered to the Atlas Centre in 1983 and was intended initially to be a migration device for datasets in the IBM MVS operating system which was in internal trial use at the time. In due course software was developed which enabled the M860 also to handle disk datasets created by IBM's VM/CMS and the Cray operating system COS. It was taken out of service in the early 1990s when a larger StorageTek 4400 automated cartridge system came into use.
The M860 could store 110 GB of data, any item of data being able to be retrieved automatically within about 20 seconds. It achieved this by writing the data on a length of magnetic tape 66 feet long and 2.7 inches wide using a helical-scan rotary head similar to those in domestic video recorders. Each tape could contain 27080 sloping tracks, each 6 inches long and holding about 6.5 Kbytes of data, giving a total capacity per tape of 175 Mbytes.
When the tape was not in use it was automatically rolled up and stored in a cylindrical cartridge about 4 inches long and one and a half inches in diameter. About 300 such cartridges were stored in a honeycomb structure to form a 'storage module' with a capacity of 55 Gbytes, and the system installed at the Atlas Centre had two such modules. The entire action of selecting a cartridge, extracting the tape, threading it into one of the read/write stations, positioning it, reading or writing the data and putting the tape away again was under automatic control.
As an illustration of how the M860 was used, by 1987 about 650 cartridges had been allocated, of which 304 were used by MVS, 215 by the Cray and 82 by CMS, all for backup and migration purposes.