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I remember using high-density disks at a pinch with an Amiga 500 back in the day, after my friend's mum couldn't get a pack of double-density media. We would tape over the HD hole (possibly unnecessarily), format on and use only with the Amiga, and they seemed to work fine. I was always expecting them to be unreliable, but I tested one such disk recently and every sector could be read without problems, three decades later (and the disk wasn't even very well looked after). Is this just one of those “theoretically bad” things?

(It seems the situation for 90 mm (3.5″) floppies is somewhat different from that of 5.25″ media, as the coercivity for HD media is only about 10% greater than DD, and both use the same number of tracks; only the sector density is doubled.)

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  • (I hope to be able to answer this question myself, at least in terms of magnetic field strength and signal quality, as I have a Greaseweazle and access to a KryoFlux and a variety of drives and diskettes...)
    – screwtop
    Commented Oct 24 at 5:24
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    Nice question, as I always thought it will not work. I cannot remember that I actually tried it, but those times are long gone. -- Good thing that you linked to the "same" question concerning 5.25" disks and reasoned why it does not apply. I was going to vote to close as a duplicate.
    – the busybee
    Commented Oct 24 at 5:58
  • I'd say you've answered your own question. There are two differences between HD and DD, head width and coercivity of the medium, but if you write in a DD drive and read in a DD or HD drive, the first is no issue, and if in 3.5" media the coercivity is close, the magnetization seems to be stable enough...
    – dirkt
    Commented Oct 25 at 4:47
  • I most certainly did have reliability problems using HD disks as DD sneaker net disks. But those were AOL disks (strike 1) previously formatted as HD and not degaussed (Strike 2). Re-formatting/writing from an HD drive (ie, smaller write head - strike 3). Not comparable to your situation.
    – Kelvin Sherlock
    Commented Oct 25 at 20:56
  • I'll be interested to see if degaussing the disks before use has a measurable effect - I do have a tape head demagnetiser which might be suitable for that.
    – screwtop
    Commented Oct 29 at 3:36

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Yes, it was possible to use a low-level reformat to put double-density tracks on a high-density disk.

I remember being told that it was a little less reliable because it left some magnetic scraps on the disk, and that compatibility got even worse when you tried to reverse it.

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