I'm backing up some old floppy disks, some of which are Double Side Quad Density 96tpi Soft-Sectored disks, which I understand to have capacities of 720KB. When I read the disks (in a 1.2MB drive) they are only being read as 360k (both when mouting the floppy and when using dd), so I need to be careful that I'm not missing out half of the data on the discs. Additionally, these discs can also be read in a 360k drive.

Eventually I'll be erasing them (probably with multiple PRNG rounds) so will also need to ensure that all data is erased.

Am I misunderstanding how the disc capacities work? How can the full 720k (if applicable) be read and written? Thanks

  • 2
    Some systems* treat each side of the disk as a separate volume.  Could that be happening here?  (* For example, the BBC Micro — though the one I had only went up to Double Density.)
    – gidds
    Jan 12 at 21:00
  • How might this be determined? Jan 12 at 22:34
  • 2
    It probably depends upon the type of system (OS, hardware, etc). Is this Apple ][?  DOS?  Atari ST?  Amiga?  Unix?  Acorn?  Amstrad?  Something else?
    – gidds
    Jan 12 at 22:56
  • 1
    What did you "understand" exactly? Nothing on that page indicates that 720 kB would be a standard capacity for any quad density disks and vice versa.
    – tevemadar
    Jan 13 at 12:20
  • @tevemadar That wikipedia page claims that "5.25 inch, quad density" might be a format at 80 tracks, 96 tpi with an unformatted capacity of 500KB/side. This is 720KB. Jan 13 at 18:27

1 Answer 1


720KB 96tpi recording and 360KB 48tpi recording use exactly the same media, it's just that 96tpi drives record narrower tracks and put them twice as close. 96tpi at 720KB is not a format that was typically used on 5.25 inch disks on IBM-PC-like systems, and is not supported by the major BIOS brands. On the other hand, 1.2MB drives are technically able to read 720KB disks.

So at the moment you might wonder whether your disks do contain 720KB and your drive / driver is misrepresenting them as 360KB disks and skips half of the data, or whether there is actually just 360KB on them. The label "96tpi" on the disk itself is not worth anything. You can perfectly format a "DSQD" disk at 360KB DSDD, and use it as a 360KB floppy.

I am very confident that your disks only contain 40 tracks (which fill the complete medium at 48tpi) and not 80 tracks. While formatting a disk, every sector is prefixed with a "sector ID" header that does not just contain the sector number, but also the head number and the track number. If your disk would be formatted with 80 tracks (96tpi), the first track would be labelled "track 0", the second track "track 1" and the third track "track 2". If you try to read a disk like this as 40-track (48tpi) disk, the first track will line up and can likely be read without issues, but when you seek one 48tpi step inwards to where you expect the second track of a 48 tpi disk, you will skip over the second track of an 80-track disk and actually read the third track of it. The floppy controller chip is going to detect the mismatch between the expect track number ("1") and the track number obtained from the floppy disk surface (which would be "2"). So I can be confident that if you are able to read the disk as 360KB floppy, it will not contain 720KB.

  • "Fill the complete medium at 48tpi" Would a correct understanding then be that a 48tpi PRNG pass of a disc formatted to 96tpi erase all data? Essentially that a 0xff write command at 48tpi would mean a 96tpi read command would give two lots of 0xff? Jan 12 at 14:58
  • I wonder what fraction of 48tpi (0.0208" spacing) drives would be able to read data written at 96tpi? Even if a drive's head picks up a small amount of signal from adjacent tracks, such noise would be below the level of the signal on the selected track, and automatic-gain-control circuitry which self-calibrates to the strength of the selected track should filter it out.
    – supercat
    Jan 12 at 16:57
  • 2
    @SyntheticAscension Use a floppy disk degauser and it won't matter what formats were ever used on those disks. Then reformat them, if you ever intend to use them again.
    – jwdonahue
    Jan 13 at 2:27
  • @SyntheticAscension Floppy drive heads are more complicated: When you write a track, the head also erases the vincinity of that track to make sure no remnants of the previous contents remains. The erased zone between tracks of a medium written by a 48tpi drive coincides with the extra track an 96tpi drive woud write. So you wouldn't be able to pick up the same data twice, even if you disregard the issues with the floppy controller expecting to see formatting information ("sector IDs"). Jan 13 at 8:31
  • @superrcat: It is known that 48tpi drives can read disks that were bulk erased and then formatted and written in an 96tpi drive in "double stepping" mode, skipping all the odd tracks. It is also known that overwriting data on a 48tpi medium using a 96tpi drive does not sufficiently erase the old contents seen by a 48tpi drive, and reading the updated medium in an 48tpi drive will often fail. As my experience is mostly from PC systems that don't use DD data rate on 96tpi disks, I don't know whether writing the odd trakcs will erase enough around the wider 48tpi tracks to impmrove readability. Jan 13 at 8:36

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