I've been thinking about something obscure to post here, and I think that
I've found just the thing: the 720k 5.25" QD (quad density) disk, a format
that both never and always existed on the PC.

What is this format? Well, if you hark for a moment back to the days of the
Z80 and CP/M, 5.25" 40-track drives were the standard among CP/M machines. Sure, different logical formats were used between CP/M machines, but most of the drives used the same 5.25", soft-sectored, 48 TPI, 40-track drive type.
And all was good.

But you know what? The storage space on these floppies just kinda really
sucked real bad. It wasn't uncommon for your average CP/M machine to have
only maybe 200-400K of space to work with on a double-sided, double density 40-track disk. This was fine in 1978 when Shugart came out with these drives for the first time, but by 1981 or so it was getting sort of restrictive for CP/M purposes. Users made their own boot disks often, and after reserving a couple of those 40 tracks to the file system and CP/M boot info, and after copying half their handy utilities to that disk, users would often find that their disks were full. So, how do we squeeze more info on a disk? Why, by halving the size of the read-write head!

Yes, that's right: Manufacturers had the idea to shrink the drive head to a
96 TPI system *before* DSHD disks existed. All that the computer then did
was format the same media with the same number of sectors per track but with
80 tracks instead of 40, resulting in a double-double-density (quad density) format.

Why haven't you heard of this? Well, in the PC world, IBM had a sole market share for the first little bit because they were legally active through
about 1986 in making sure clones didn't appear. So, barring clones, you had
two choices of disk drive for your IBM PC or XT: 40-track single-sided, and 40-track double-sided. That was it, but it didn't necessarily matter too
much because early IBM software ran off its own floppies, meaning users
didn't make "work" and "boot" disks like was so common in the CP/M world.
You just didn't do that in DOS unless you had a large hard disk.You had
maybe one floppy with your documents on it, and for that 360K was more than enough space.

Then, the IBM PC AT introduced the DSHD format to the PC world with 96 TPI,
a higher-coercitivity magnetic media, 360 RPM, and all that jazz. Notice something? That's right: *the DSHD format is 96 TPI as well.* But the 2*360K=720K format didn't catch on in the PC world because it was now superfluous: 1.2MB was far more than 720K, and so the 300RPM 96TPI double-double nonsense just never made it into the hands of PC users.

This doesn't mean the format wasn't completely excluded from MS-DOS or even
the BIOS; it just wasn't presented as a default option for users, especially when you consider the fact that 1.2MB media was now available and IBM needed
to sell ATs. However, Microsoft still snuck QD support into MS-DOS's IO
system and the FORMAT command, and it's still possible to format and access 720K QD media as late as DOS7 and Windows 98(!). Do be aware that these
steps may not work on certain BIOSes, and it is entirely possible to apply
too high of a write current and "burn" HD signals onto DD media using these methods, making them uneraseable save for demagnetization. *Do this only at your own risk.*

First, you need a 1.2MB drive for your PC. While the disk format was
supported by DOS and the BIOS, the drives were all manufactured using the Shugart interface, incompatible with PCs (because PCs never used these
drives). I have tried a QD drive in a PC, it doesn't work with either of my standard floppy controllers. So, use an HD drive since these also are 96 TPI and were supported by PCs and are more likely to work with a given floppy controller.

Next, you need to format the target disk using the usual 360K format first. This step is very important: it marks the boot record of the floppy with information that tells DOS that the media inserted needs to be read at 300
RPM and written with a lower write current. DOS will see this information during the next format step and set the drive parameters to match, making
sure that our media will be written correctly. You cannot skip this step;
this is important to ensure that the media does not get written with an HD signal.

Finally, issue a "FORMAT /T:80 /N:9." DOS will read the boot sector and
check if an existing FAT format is there, and see the media information. It will then tell the drive to spin at 300 RPM and use the DD/QD write current. When it finishes, you will have a 720K QD disk, ready to be used with DOS.
Of note is that the logical layout is identical to the 3.5" 720K format,
with 2 sectors per cluster and 713 clusters.

Enjoy this little nugget of a forgotten, always-there-but-never-there
format, and keep in mind how close CP/M was to being *the* PC operating
system as you do. :P