Subj : Re: floppy disks
To   : boraxman
From : StormTrooper
Date : Fri May 02 2025 08:03:53

 bo> I know for 3.5" discs, there was a small difference in the thickness of
 bo> the actual disk.  Higher density disks had I think a thinner magnetic
 bo> coating, as it had to be more sensitive.  Even DD discs written to in HD
 bo> drives, could have problems if read on a DD drive subsequently.  Im
 bo> guessing subtle differences in the sttrenght and size of the magnetic
 bo> field, and in the responsiveness of the magnetic layer made HD discs
 bo> unusuable in DD drives.

Most of the problem with HD drives writing DD floppies is the phyiscal size of the head... presumably it writes straight down the middle of the track, if its been written on a DD drive previously you can have noise surrounding the new track, while the DD drive will see the noise on the full width track.

The HD floppy has a coating that requires a stronger magnetic field to coerce the data you want written to it.  If you put an HD floppy in a DD drive, then you can expect problems as the DD drive doesn't produce as much magnestism, basically meaning its more likely to not change the sector enough and it'll revert or change in some way.  However, I have some IIgs system images written inadvertently to HD3.5" floppies and have had no problem with them over many years, so its really a case of your mileage may vary.  

ST

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