the nine are available to the computer at any one time. The ninth drive is there for a spare for the user and can also be worked on "offline" by a Field Engineer while the other drives are in use by the customer. Each drive's system address is determined in part by a user-swappable plug, one such plug denoting a spare drive not system accessible. This permits physically changing the address of a drive by changing the plug. A 2844 Control Unit can be added to the 2314 Control Unit which allows two S/360 Channels simultaneous access to two separate disk drives in the Storage Facility. Other 2314 models came later: IBM 2314 direct access storage facility - A series In 1969 IBM unbundled the facility into separate models allowing up to nine drives (eight on line) attached to a 2314 Storage Control: 2312 Disk Storage, a one drive module. 2313 Disk Storage, a four drive module. 2318 Disk Storage, a two drive module. IBM 2319 In a response to competition from plug compatible manufacturers of 2314 equivalent storage subsystems, IBM beginning 1970 introduced a series of low priced three drive module 2319s which were manufactured by removing one module from the four drive module 2313, rebranding it as a 2319 A1 and offering it at a substantially reduced rental price. This had the effect of lowering the rental price to new customers while keeping the high rental price on existing customers. The 2319-A1 attaches to integrated controllers for only the System/370 Models 135 and 145. Conventional 2314 DASD such as the 2312 or 2318 can attach to the 2319-A1 2319 B series of three disk drives modules allow three, six or nine drive attachment to a new 2314 Model B Storage Control Unit. IBM 3310 IBM introduced the IBM 3310 Direct Access Storage Device on January 30, 1979, for IBM 4331 midrange computers. Each drive had a capacity of 64.5 MB. The 3310 was a fixed-block architecture device, used on DOS/VSE and VM, the only S/370 operating systems that supported FBA devices. IBM 3330 The IBM 3330 Direct Access Storage Facility, code-named Merlin, was introduced in June 1970 for use with the IBM System/370 and the IBM System 360/195. Its removable disk packs hold 100 MB (404×19×13,030 bytes). Access time was 30 ms and data transferred at 806 kB/s. A major advance introduced with the 3330 was the use