A row of Xerox disk drive units. Notice the empty blue plastic drive covers sitting on top. The disk packs consisting of a stack of platters are dropped into the machine, the door locked and the disks spun up and read. Kind of constructed like a washing machine (but much more expensive).
Each pack could hold maybe a couple hundred megabytes. Same principal as a current hard disk drive except the magnetic platter(s) are contained in a sealed case and not removable.