Expensive, yes! Selectrics performed very well and were popular in business and in academia especially because you could change fonts by changing the "golf ball" type head. They also could be a bit costly for supplies because most used a single-use ribbon cartridge that gave beautifully sharp type but only used once through the cart. Multi-use fabric ribbons also were an option.
Actually, the "Winchester" design of hard drives was new in the '60's, but is still the basic design concept today. The unique feature was the idea that clearance of the head above the spinning hard drive disk surface was NOT achieved by supporting the head rigidly over the surface with precise tolerance. Instead, it depended on the phenomenon that the rapidly-spinning disk surface created a flow of air following the surface, and this flow could support the head if the head were held over on a flexible arm. The resulting clearance could be VERY small and self-adjusting for small variations in the disk surface and did NOT require very precise arm construction. The small clearance also made for strong magnetic coupling, allowing faster data transfer. The only restriction (easily met) was that this all worked only when the disk is rotating, so starting and stopping could be done only when the head arm was retracted to a "parking track" location where a "head crash" (contact with the surface) was tolerable.
I do not remember whether this hard drive design was in use in that mainframe system in the late 60's. I do know that an earlier form of random-access storage was in use. There were a few large floor-standing machines. Into each one could place removable "disk packs". This unit was a set of many rigid double-sided magnetic disks about 12"diameter, similar in size to an LP phonograph record, all mounted on a vertical shaft perhaps 10" high. The entire pack was stored in a case to keep it free of dust, and removed from that to place into the drive unit. The drive had arms for each set of heads which positioned them rigidly at fixed clearances over each disk surface. The disk pack rotated, and the arms moved to track locations on the disks just as current HD's do it. These units did NOT use the "air cushion" technology of Winchester drives, but did have the feature that the interchangeable disk packs could allow each drive to serve very different data sets. For example, in the particular case I cited above, the interactive APL system could not be run under the normal OS of the mainframe. So to get it up for users, the disk pack for APL operation was mounted in a drive and the entire mainframe switched over to running that OS and language for a scheduled time period. Then the process was reversed back to "normal" multi-use operations under a different OS. I am not sure what the storage capacity of one disk pack was. In the system I used, each user was allotted several workplaces of 32 KB each, and there surely were a few hundred users' worth of data stored. In any one workspace a user would store their software and their data as named variables, and could copy programs or data variables from one workspace to another.
For those who never encountered it, APL was an acronym for "A Programming Language" (how original!) and was very mathematics-oriented. All data were basically vector arrays (n-dimensional!), so simplest data were a single constant (0-dimensional), a vector of several values (1-dimensional), a flat array (2-dimensional). Larger arrays merely had more dimensions and therefore indices. There were lots of symbols (mostly Greek letters) used for pre-defined operators you used to write your own custom programs or just simple do-it-now calculations. Almost all of these were designed to operate on scalar or vector variables, but the built-in functions in general used matrix algebra techniques suited to n-dimensional arrays. Named pre-written programs could be called as subroutines just by specifying their name and the numeric arguments they were to operate on. It had lots of power and features many non-math people did not use or understand.
Oh, by the way, the common Line Printer used in computer centres at that time was NOT dot matrix. It printed a lot faster than those, and used a different technology. If you want a description, post back here. This is NOT related, of course, to your original post.
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