This is an example of Diablo Corporation's Model 31 removable cartridge disk drive. The disk proper was 14 inches in diameter constructed of an aluminum platter coated with magnetic oxide. This platter was then enclosed in a plastic shell that formed the pack.
The device used a hard-sectored format (sector demarcations were cut slots on a protruding rim near the hub) and had a basic storage capacity of 1.2 million 16-bit words (~2.5 Mb). The number of sectors was twelve per track. The disk was double sided, hence two read/write heads were employed in the drive. As in "modern" hard disk practise the heads "fly" on a layer of air and never touch the surface of the spinning platter. Two hundred and three tracks were written on the disk to each of which the head carriage could be commanded to move to.
This is a view of the positioner mechanism; the heads are visible to the lower left. The disk on top of the positioner is the rotary transformer which tells the drive what cylinder it's set to.
This basic drive was used as the basis for Data General's model number 4047 as well as, I think, Digital's RK-03 (again, corrections are welcome).
The specimen shown here is my "spare" drive (the other two being installed
in the Nova 840
). It is functional in all
respects save that it has a burnt out door release solenoid. I'll probably
wind another core for said device when I get time, thereby restoring the
machine to operable status.