When I was in the Navy, working as an instructor for the F-14 Tomcat Fleet Replacement Aviation Maintenance Personnel (FRAMP) training command in Virginia Beach, VA, we had a base-wide mini-computer. It was in the same building as our FRAMP school, and I became friends with the DP (Data Processing) Chief and several of his workers, so I used to go into their office all the time.
The mini-computer was a PDP-11/78 or 11/780,I can't quite remember which, and it had two large hard drive devices over in the corner. I believe the designation on them was RP-06 or RP-O6. They looked to be the size of a washing machine, and the top lid opened by sliding a door inward. You could remove the disk pack by putting a domed device inside, screwing it down then removing the stack. The platters looked to to be 12 to 14" in diameter, and it had several in a stack.
There were two of the drive devices, and I they would back them up by replacing the stack in one device, then backing the other up to it, then repeating the process in reverse for the other stack.
Btw, this is when I first got interested in computers. It was in 1982 - 1985. I got my first computer, a VIC-20, in mid to late 1982.