You no doubt have learned to conserve storage space, memory and code. I didn’t get into computers until the early 80s, before hard drives. It still amazes me that gigabytes are so cheap (they’re talking about petabytes now!).
Actually, mainframes did have disks back then. The removable 1316 disk unit had 6 14” platters with 10 usable surfaces, 200 daqta tracks, a capacity of about 7.25 megabytes, and weighed several pounds.
For use, it was mounted in a 2311 disk drive unit, which in addition to the spindle had 10 heads mounted on a hydraulically driven carriage that moved radially across the surface of the disk. Because the heads were part of the drive but the disks were separate and interchangeable, alignment and positioning of the heads was a very critical process.
It was superceded by the 2316 disk, which had 11 14” platters and 20 surfaces with 200 data tracks, but double the bits per inch on each track, for a capacity of over 29 megabytes. The new drive was called the 2314 or 2319. At first it was available only in an array of 9 units, including one spare. Later versions were available in smaller quantities.
The 1316 and 2316 disk units were quite fragile, and were easily damaged by mishandling. The damage might not be apparent, and became so only when the disk was mounted and started, at which time it would destroy the carriage mounted heads on the drive. I had multiple instances of having to replace the same set of heads on all 9 drive units in a 2314. This took hours because of the critical alignment procedures, which required a stabilization (warmup) period after head replacement before even starting.
Today all disks are sealed units with dedicated internal heads and carriages that never require alignment - the tracks are located and formatted after the heads are married to the platters. And who remembers disk magabytes any more?