I'm sorry, Vince, if you thought my post about 2311s was written competitively, following Pyan's entry on 5440s. I was actually trying to add a bit of context. Py's picture didn't seem to me to really show the physical size of the disks in those days, nor the limitations of their use, which I thought was important.
The difference between the 2 models of disk is far less important than their similarity. The head contention (spreading files over disks) issue I also thought might interest people. It was a large factor in system design at the time.
The NCR Century range (1960's) had a drive with twelve heads per surface (on a single arm) so the head movement was much reduced. The one thing that only came to light after a couple of years was that the drive was subject to twelve times more head crashes than a normal drive. It was very fast though.
NCR also had a device called CRAM. Strips of magnetic tape were stored on mylar cards (256 or 512 cards to a deck). The removable deck was held vertically over a drum and each card could be addressed individualy and released to drop and wrap around the drum and then be accessed by a set of heads. Sounds complex? Sounds too complex? It was. A common fear amongst operators was a 'double drop'. The term speaks for itself.
The CRAM drive is on the left in the picture and you can see the cradle that held the cards and was used to mount the deck in the unit.