14, 1956, as an addition to the popular IBM 650. It used the mechanism of the IBM 350 with up to three access arms and stored 6 million decimal digits and 600,000 signs. It transferred a full track to and from the IBM 653 magnetic core memory, an IBM 650 option that stored just sixty signed 10-digit words, enough for a single track of disk or a tape record. IBM 1405 The IBM 1405 Disk Storage Unit was announced in 1961 and was designed for use with the IBM 1400 series, medium scale business computers. The 1405 Model 1 has a storage capacity of 10 million alphanumeric characters (60,000,000 bits) on 25 disks. Model 2 has a storage capacity of 20 million alphanumeric characters (120,000,000 bits) on 50 disks. In both models the disks are stacked vertically on a shaft rotating at 1200 rpm. Each side of each disk has 200 tracks divided into 5 sectors. Sectors 0–4 are on the top surface and 5–9 are on the bottom surface. Each sector holds either 178 or 200 characters. One to three forked-shaped access arms each contains two read/write heads, one for the top of the disk and the other for the bottom of the same disk. The access arms are mounted on a carriage alongside the disk array. During a seek operation an access arm moved, under electronic control, vertically to seek a disk 0–49 and then horizontally to seek a track 0–199. Ten sectors are available at each track. It takes about 10 ms to read or write a sector. The access time ranges from 100ms to a maximum access time for model 2 of 800ms and 700ms for model 1. The 1405 model 2 disk storage unit has 100,000 sectors containing either 200 characters in move mode or 178 characters in load mode, which adds a word mark bit to each character. The Model 1 contains 50,000 sectors. IBM 7300 The IBM 7300 Disk Storage Unit was designed for use with the IBM 7070; IBM announced a model 2 in 1959, but when IBM announced the 1301 on June 5, 1961, 7070 and 7074 customers found it to be more attractive than the 7300. The 7300 uses the same technology as the IBM 350, IBM 355 and IBM 1405 IBM 1301 The IBM 1301 Disk Storage Unit was announced on June 2, 1961 with two models. It was