An early type of rotating mass storage, used with mainframe computers of the 1950s and '60s. Conceptually, a drum drive was similar to a disk drive; data was stored on a rotating drum that was coated with magnetic material. The drum's surface was divided into tracks and sectors, and one or more heads positioned near the drum surface recorded and read back data on the drum. Drum drives were actually used as primary memory on some computers of the era; the CPU read program instructions directly from the drum as it executed. However, by 1960, most computers had magnetic core main memory and drum drives were used mainly as fast mass storage.
Both "fixed head" (or "head per track") and "moving head" types were offered. The fixed head types had a separate read-write head for each track on the drum, while the moving-head types had one or more heads on slider mechanisms which moved across the cylinder to access different tracks.
Drum drives were massive, heavy devices that bumped up against technological limits fairly quickly. The UNIVAC FASTRAND drum drive used a six-feet long cast iron drum; it was one of the fastest and highest-capacity mass storage devices of its day. But when it ran, the drum developed so much angular momentum that it resisted the rotation of the Earth underneath it, and the drive would walk around the room and knock things over if it wasn't bolted to the floor. (Legend has it that the U.S. Army did an early experiment in mobile computing by installing a UNIVAC mainframe in a tractor-trailer truck, and the first time it took a curve at speed, the angular momentum of the FASTRAND drive flipped the truck over.) Running the drum faster sped up data transfer rates, but requried a heavier cylinder to avoid dangerous fracturing, but that exacerbated the problems with mounting and keeping the device stable. Ultimately, disk drives were able to pack a lot more storage capacity in less space and with much less rotating mass. The last drum drives were taken out of service in mainframe computing installations around 1980.