On 06/06/2016 08:28 AM, Liam Proven wrote:
  AIUI -- not sure -- the 720 kB *format* was mostly
used on 5.25"
 DS/DD 80-track disks, no? 
I'm not sure what you consider 720K, so the question is hard to answer.
 On MS-DOS machines?  Perhaps, but there weren't a lot of them in
comparison to the IBM PC and its ilk.
But IBM offered the "slimline" 3.5" drive pretty early on; see the O&A
section on it--and it was indeed a DD "720K" formatted application.
(FWIW, the earliest drives on the 5150 were used as 160K and 320K format
implementors.)
   If a good
150RPM ED drive were to have been readily available, then
 2.8M could have been retrofitted to all 1.4M systems, including
 Amiga, etc. But would that, and the Barium-Ferrite disks have been
 worth it for just twice the capacity? 
 Wouldn't they have got much cheaper if every cloner had used them? 
 
Slow-spinning floppy drives are at best, a kludge.  Like a lot of other
physical phenomena, the energy induced in a read/write head is roughly
proportional to rotation speed.  Low speed -> low output.  So not such a
good idea.  I liked the idea of the 600 RPM 3.5" Sony drives.
--Chuck