I see I remembered some details wrong, including the number of platters (it’s 6, not 5).
Quoting the IBM article:
The IBM 1311 Disk Storage Drive provided storage for 2 million characters. Developers of the 1311 engineered twice the recording density of the IBM 1301 Disk Storage Unit by reducing the space between the head and the disk by about a factor of two.
The 1311 used the IBM Disk Pack (later designated the IBM 1316 ), an interchangeable package containing six 14-inch-diameter disks in a four-inch stack, weighing 10 pounds (seen above in the man’s left hand). Each disk surface contained 20 pie-shaped regions. Sectors were segments of track lying within a region, and were the smallest addressable unit, with a capacity of 100 characters. Average access time to any sector was 250 milliseconds, which could be reduced to 150 milliseconds with an optional direct-seek feature. The disks were rotated at 1500 rpm, tracks (50 to the inch) were recorded at up to 1025 bits per inch, and the usual head-to-surface spacing was 125 microinches. The ten recording surfaces provided in normal usage a storage capacity of 2 million characters, the equivalent of approximately 25,000 punched cards or a fifth of a reel of magnetic tape.
We had the “direct-seek” feature, which as I remember it meant the heads didn’t return to the outer edge before starting the next seek.Â Note those unitsâ€”some of you are perhaps old enough to remember milliseconds!
Only 1,500 RPM.Â It’s gradually gone up, so that enterprise high-performance rotating disks today are 15,000 RPM (but lots of people needing that kind of performance are using SSDs instead).
As I recall the 1401 didn’t use the fixed sectoring; or maybe what we used was layered on top of that or something. It’s been a while now. My memories aren’t precise, but we read records considerably bigger than 100 characters as a single operation, and didn’t have to specify the size on each read (I think it was formatted into the pack somehow).