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3.5" floppy disks, in the transition from 720K to 1.44M, changed the actual coating to a different compound that was magnetically 'stiffer'. This was necessary to support the higher density, but meant the old disks could not support the new high-density format even if used in a new drive. A more subtle and much nastier problem: if you formatted a new disk in an old drive, everything would seem fine, but the information would not have been recorded strongly enough, and might fade over the next few days.

5.25" floppies made several format transitions that together accomplished an order of magnitude capacity increase until the final 1.2M format. Did any of these transitions involve a similar change in the actual coating? Or did the disks themselves stay interchangeable apart from issues of hard versus soft sectors?

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  • I had 3.5 and 5.25" HD Floppy drives in 1989 in a brand new 286, and friends had a variety of 720k and 360k drives. I don't recall ever having read problems other than when accidentally taking a 1.2 or 1.44MB disk to someone who didn't have a HD drive. So it was possibly dependent on the quality of the non-HD drives used to do the reading.
    – Criggie
    Mar 5, 2019 at 11:16
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    In a pinch you could punch a hole in the top of a 3.5" DD disk to turn it into a HD disk. It would generally work to transfer large files, but you knew never to really rely on it lasting.
    – JPhi1618
    Mar 5, 2019 at 16:37

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The main technical parameter for a floppy disk's coating is its coercivity, i.e. the resistance of ferromagnetic matter to withstand demagnetization. Coercivity is measured in Oersted, after Hans Christian Ørsted, a Danish physicist who discovered the magnetic impact of electrical current.

  • 5¼" disks storing 360K and 720K (SD and DD) used a coating with a coercivity of 300 Oersted.
  • 5¼" disks storing 1.2M used a coating with a coercivity of 600 Oersted.

So yes, the coating was changed to support the HD format.

The difference between 5¼" DD and HD coating is much larger than the same difference between 3½" DD and HD media - which are 660 Oe and 720 Oe, respectively.

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    Do you have any idea why the difference in 3.5" was smaller? Is it because DD had already a higher than needed coercivity (by the way, you have a typo there) from the beginning? Does that also mean that they could have produced DD-compatible drives with higher capacity instead of introducing a slightly different coating?
    – Selcuk
    Mar 5, 2019 at 2:30
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    @Selcuk: The challenge has always been bit density, i.e. how many bits can you store per mm2. Since 3 1/2" disks are a lot smaller (area varies with the square of the diameter), even their initial 720KB capacity required a bit density on par with the 1.2MB size 5 1/4" disks. As for the 1.44MB format, that's mostly due to MFM encoding.
    – MSalters
    Mar 5, 2019 at 13:58
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    @MSalters The much smaller size of 3.5" disks compared to 5 1/4" would have been my explanation as well. Also, if you look at the percentage of the overall disk surface area actually reachable (thus, used) by the heads on a 3.5" disk vs. a 5 1/4" disk, that is way lower for the 3.5" floppy.
    – tofro
    Mar 5, 2019 at 16:26
  • According to Wikipedia 5.25" "DD" disk capacity was 360K (a 800K variant existed); while "QD" was 720K. My second computer used 360K disks so I was very confused when I read your answer saying 360K was "SD" and not "DD".
    – Martin Rosenau
    Mar 6, 2019 at 6:23
  • SD Disks existed even in the early IBM compatible world. 160KB (or 80KB if single sided). IIRC, the double-height drives certainly could handle these...
    – rackandboneman
    Sep 5, 2019 at 8:36

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