Disk

Magnetic media didn't have to be a long tape that could only be accessed one piece at a time. Magnetic drums came early, large cylinders whose outer edges were coated with iron oxides and spun. Disks replaced them once servomechanisms that could read a sector in any order and media to hold oxides could be developed at a low enough cost. Disks were huge arrays, then they shrank to 8 inches in diameter, then 5 1/4 inches, and today are more commonly 3 1/2 inches of media encased in a plastic shell. Compact disks are another medium entirely - a single line of non-magnetic data sealed forever in thick clear plastic, incapable of being rewritten.