 Subsequently there was a period of about 20 years in which other technologies competed with disks in the secondary storage marketplace, for example tape strips, e.g., NCR CRAM, tape cartridges, e.g., IBM 3850, and drums, e.g., Burroughs B430, UNIVAC FASTRAND, but all ultimately were displaced by HDDs.Today SSDs compete with HDDs in the marketplace.The IBM 1301 Disk Storage Unit, announced in 1961, introduced the usage of heads having self-acting air bearings (self-flying heads) with one head per each surface of the disks.