They are much closer together in coercivity than 5.25 media, but it is not the same and can be unreliable. The physical disk is actually half the thickness as well.
I've heard people claim that HD disks are "higher quality" and you should write DD data on them because it will actually be more reliable, but that is complete horseshit. Those that have said that (JD) - you know who you are... say no more.
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