My father worked IT with a medical record imaging system that is very similar to what you're describing. They called them platters, and they were like CDs except the size of a dinner plate or so if I remember correctly. And there was basically a giant jukebox system to switch them out.
Sounds like a proper old WORM drive with integrated media changer - had seen one in telco I worked at, it's also referenced by Plan9 file server which used one as its main storage.
