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

Jean-François Fabre
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Matthew
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  • While it doesn't on 5.25, it does for 3.5 - depending on media and drive. – Raffzahn May 28 '22 at 18:23
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    Formatting an unformatted disk actually might appear to work. But not for long. – tofro May 29 '22 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 Jun 14 '22 at 06:57
  • @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 Jun 14 '22 at 11:17

1 Answers1

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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)

Raffzahn
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Marc Wilson
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    I think a little background on what coercivity means would make this a much better answer. – Euro Micelli May 28 '22 at 05:16
  • 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 May 28 '22 at 16:43
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    I added a definition of coercivity. – Marc Wilson May 28 '22 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 May 28 '22 at 17:08
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    @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 May 28 '22 at 18:07
  • The coercivity issue could be overcome by heating the disk beforehand to just below the Curie temperature. Or at least sufficient so that the weaker DD drive can reliably set the magnetization. Not sure though if all involved materials can stand the required heat. – laolux May 30 '22 at 02:06
  • @laolux Heating the disk to Curie temp is a sure way to delete it completely (which is used to safe-erase magnetic media in military, for example). If you do the heating only on specific spots, you have an MO drive. Unlikely that's gonna work in a normal drive. – tofro Jun 11 '22 at 06:55
  • @tofro Heating to below Curie temperature, keeping enough distance so that the disk is still ferromagnetic. Of course, data will not be very stable/long lived at that temperature, but you only heat right before writing and then let the disk cool down. – laolux Jun 11 '22 at 08:59
  • @laolux: I wonder if it would have been possible for a "copy protection" company to engineer a floppy that would be partially writable with normal equipment, but needed to be mastered with special equipment? I just saw a video about that having been done--once--with CD-R media, but it could be helpful to allow copy-protected programs to store configuration information on their boot disk without the risk that the software might get accidentally overwritten. – supercat Jun 11 '22 at 17:21