Following a development effort that began in the summer of 1969, the IBM 3340 disk unit was introduced in March 1973 with an advanced disk technology known as "Winchester."* The first 3340 shipments to customers began in November 1973.
The 3340 featured a smaller, lighter read/write head that could ride closer to the disk surface -- on an air film 18 millionths of an inch thick -- with a load of less than 20 grams. The Winchester disk file's low-cost head-slider structure made it feasible to use two heads per surface, cutting the stroke length in half. The disks, the disk spindle and bearings, the carriage and the head-arm assemblies were incorporated into a removable sealed cartridge called the IBM 3348 Data Module. A track density of 300 tracks per inch and an access time of 25 milliseconds were achieved.
The 3340 offered the optional availability of fixed heads, which provided an average access time of only five milliseconds. It had three types of data modules: 35 megabytes, 70 megabytes, and 70 megabytes of which 0.5 megabyte were accessible with fixed heads.
Two-to-four 3340 drives could be attached to the IBM System/370 Model 115 processor -- which had been announced concurrently with the 3340 -- providing a storage capacity of up to 280 million bytes.
* Some observers have noted that the 3340 was known as "Winchester" because its development engineers called it a "30-30" (its two spindles each had a disk capacity of 30 megabytes), the common name of a rifle manufactured by the Winchester Company. Kenneth E. Haughton, who led the 3340 development effort, is reported to have said: "If it's a 30-30, then it must be a Winchester."