Very interesting.
I think the point of a smaller diameter platter and a more powerful motor is to spin the platter from zero up to max RPM instantly. Smaller diameters have less rotational inertia. I believe this affects seek time.
Did you ever do anything with an Altair? I’ve heard their last version was sometimes equipped with a hard drive or two as add-ons in addition to a pair of 8” floppy drives.
Rotating mass storage devices don't spin down, unless they are powered off. Some OS'es do so, if the power settings are set appropriately. Spinning up to RPM "instantly" is a massive amount of torque, even for the smallest disk, and it's not something you want to do repeatedly.
Rotational latency is determined by the speed of the disk, once it has stabilized. 4.2 milliseconds is the time it takes for a disk to rotate 180 degrees at 7200 RPM. That's where the "average" comes from: the length of time that you have to wait for a disk to rotate into position is -- on average -- 1/2 the time it takes to make a full revolution.
Did you ever do anything with an Altair? I’ve heard their last version was sometimes equipped with a hard drive or two as add-ons in addition to a pair of 8” floppy drives.
No, the stuff I worked on would fill a room, and heat several houses.