Is is possible to format and use 5.25 high density disks in a double density drive?

  • While it doesn't on 5.25, it does for 3.5 - depending on media and drive.
    – Raffzahn
    Commented May 28, 2022 at 18:23
  • 1
    Formatting an unformatted disk actually might appear to work. But not for long.
    – tofro
    Commented May 29, 2022 at 12:15
  • @Raffzahn: is that because the 665 oersteds are "close enough" to the 720 required for 3.5" HD, whereas you need double the grunt for 5.25" HD (600 rather than 300)?
    – paxdiablo
    Commented Jun 14, 2022 at 6:57
  • @paxdiablo Jup. Media sensitivity as well as drive power do vary (this is the real world :)) It's much like using 220V appliances (continental) at 240V (GB) during the pre switching PS era. Usually it works - except when it doesn't :))
    – Raffzahn
    Commented Jun 14, 2022 at 11:17

1 Answer 1


No. The coercivity of the media is different. HD disks are higher coercivity, so a normal DD drive would not have the requisite head strength.

Coercivity of floppy disk types:

Size Density (Usual) Formated Capacity Coercivity
8" all formats 300 oersteds
5.25" double density 360 KB 300 oersteds
5.25" quad density 720 KB 300 oersteds
5.25" high density 1.2 MB 600 oersteds
3.5" double density 720 KB 665 oersteds
3.5" high density 1.44 MB 720 oersteds

(Table taken from the Wikipedia entry about Floppy Formats)

Coercivity, also called the magnetic coercivity, coercive field or coercive force, is a measure of the ability of a ferromagnetic material to withstand an external magnetic field without becoming demagnetized. Coercivity is usually measured in oersted or ampere/meter units.

(from Wikipedia)

  • 10
    I think a little background on what coercivity means would make this a much better answer. Commented May 28, 2022 at 5:16
  • If a high-density disk that started out degaused was written (possibly with a custom controller) in a format that could be handled by the low-density drive's electronics, would there be any problem reading it in a low-density drive? I would think it would be possible to produce enhanced-density read-only disks for platforms like the Commodore 1541 or Apple //c.
    – supercat
    Commented May 28, 2022 at 16:43
  • 2
    I added a definition of coercivity. Commented May 28, 2022 at 16:56
  • In other words, higher coercivity makes the material more stubborn to changing what is recorded. That's good for longevity, but you need a strong disk drive to actually change it when you want to.
    – Dan
    Commented May 28, 2022 at 17:08
  • 2
    @DanSheppard [extreme simplified] It's not really about longevity, but the 'size' of a 'bit'. The strength of a magnetic field falls rapidly toward the 'sides', thus higher coercivity allows 'smaller' 'bits'. With shrinking 'bit' size, the energy needed to flip also shrinks, making HD not really better on the long run.
    – Raffzahn
    Commented May 28, 2022 at 18:07

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