Back in the olden days, like 1995, classmates used to buy cheaper DD floppies and drill holes into them to use them as HD floppies (the 3 1/2 floppy had a hole in one of the corners to allow the drive to distinguish between DD and HD drives).

I didn't do that because I feared data loss, yet in a recent discussion, people claimed that this was not much of an issue. Which led me to reconsider; I assume that manufacturers switched to producing only HD media, simply putting some of them into DD casings, possibly those who didn't pass as well in QC. From an economic perspective, it would make perfect sense: rather than having to manufacture two different media, the manufacturer could produce only one. The cost difference between DD or HD media would vanish over time and far outweigh the expenses saved by having to maintain only one production line.

However, someone else claimed that there is a substantial physical difference between DD and HD media, which is substantial enough to necessitate the production of "true" DD media. Therefore, people "faking" HD disks would really risk data loss.

Is he right or not?


2 Answers 2


Yes, there's a substantial difference between HD and DD media.

Magnetic material has a property called coercivity that is measured in oersteds. The coercivity of the magnetic coating is related to the field strength necessary to change the magnetization when writing data.

For 3.5 inch floppy disks, DD media has 665 oersteds, while HD media has 720 oersteds. [1] So when using the wrong media, writing data will either use too little or too much field strength, resulting in "weak" bits that are difficult to read and can produce errors, and also reduce the lifetime of data.

The same kind of difference (but with a different numbers) exists between HD and DD media for 5.25" floppies.

[1] http://www.retrotechnology.com/herbs_stuff/drive.html

  • By that definition, HD drives should be rather more coercive in writing HD tracks to DD disks, resulting in strong bits, not weak bits. Whereas writing DD tracks to HD disks would fall short. The only other differences are the doubled track density and the therefore half-as-wide write head reading and writing to the media, which is obviously not affecting anything, reading and writing the same media in the same HD drive. Aug 16 at 18:00
  • @HenrikErlandsson HD drives could detect DD floppies (notches are different) and adjusted writing strength automatically, so this is mostly academic, but yes, using too strong a current in the write head causes weak bits because while the current bit can't get "stronger", it influences the previous, already written bits and makes them weaker. Which is why you use just the right current, and not the strongest current possible (which would have been easy to do). Different width of heads isn't related to media, which was the question.
    – dirkt
    Aug 16 at 18:13

Since you say drill, you mean 3.5".

You ask several questions.

DD and HD are close in coercivity, and so an HD drive should be able to write to a DD disk fine, in fact more than fine. There would have to be something wrong with the coating for it to fail.

There are no physical tracks in the coating of a floppy disk, only the distance between tracks - this is functionally defined as the width of the point coerced without coercing the adjacent tracks, since it's just a coating.

This means that after a successful write, HD and DD are equal for reliability. Only storage, mechanical damage (such as passing it through drive heads numerous times, even for reading), and active magnetic fields can change the information once coerced.

This means that your classmates are right.

As for the second question, the "someone else" is actually claiming the opposite: a DD drive could not coerce the coating of an HD floppy disk reliably, unless it was overspeced. The initial write would be too weak.

Now, this would be up to read tolerances and again there's a functional definition. The hole in the plastic shell is obviously irrelevant to all of this, since it's just for a sensor to detect. A DD drive would not even look for it. It would just read the disk, find an ambiguous-strength write, and round the 1 or 0 the wrong way, resulting in a read error. And this still doesn't mean the information has been altered by the above storage factors (and so could well be recoverable!), just that it was not written reliably enough in the first place, for drives that fulfilled spec.

So, the "someone else" is also right.

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