non-PC Floppy imaging

[Eric Smith]

On Fri, Jan 5, 2018 at 3:45 PM, Dave Wade via cctalk <cctalk at classiccmp.org>
wrote:

> IBM invented the 8" floppy disk format. Generally their disks follow the
> standard 3740 format.
>

True for anything you're likely to encounter in the "real world", but in
the interest of muddying the waters I'll point out that IBM's _first_
floppy drives, used for microcode load on big iron, were NOT even remotely
compatible with the later 3740 and succesors. The disk was the same
physical size, but the index hole was near the edge of the disk, rather
than near the spindle. They spun at 90 RPM rather than 360, and were
read-only.  (Obviously IBM had some drives that could write that format,
but they didn't provide them to customers.)

I think it's a safe bet that the 4331 microcode disks do NOT use that
format. Guy would have noticed if the diskettes didn't look like "normal"
8-inchers.