@kenshirriff 'My time' started about 10 years after this when I had unrestricted access rights to machine rooms. Exchangeable disc packs were common, there were multiple read/write heads which operated in parallel, sequential sectors were interspersed to allow for processing during rotational delays. You organised data in cylinders to avoid moving heads and in vertical 'stacks' (can't remember the actual term) for fastest R/W speed.