Thanks guys - this is the information I was looking for. Does this summarize it ?
The magnetic coating on a 3.5" HD floppy is calibrated to record with a higher charging current than the coating on DD floppies.
A drive that detects a DD disk will only write at the lower current standard for DD disk magnetic coating.
As a result,
a) the new written DD signal will be weaker than specified for HD medium
B) any data or format information previously written at HD current level will not be fully erased by a DD write operation
c) the ghost of previous HD data or format may last longer than the DD signals that are written over it.
Logically, if a more thoroughly magnetized HD "ghost" signal persists in the coating medium, it could over a period of time re-magnetize the weaker DD signal, the way any permanent magnet will do. In fact, this would happen even if the disk is bulk-erased or de-gaussed before a DD write, because whatever polarity is written as a particular DD signal point will eventually be affected by polarity of neighbouring molecules that did not take up the weaker DD charge.
I can see that this is a greater problem for diskettes than for CDs. CD/DVD signals are recorded as binary on/off optical bits, so there is quite a wide range of tolerance for stronger or weaker laser reads and writes. But floppies record a frequency modulated (FM) signal with a carrier frequency that is much higher than the data rate. This would be very prone to failure as the signal fades or degrades, as we know from floppies in long-term storage.
5.25" floppies retain data generally longer than 3.5" floppies - I have some that are 35 years old and still can be read.
I've ordered some DD disks.
Rick
The magnetic coating on a 3.5" HD floppy is calibrated to record with a higher charging current than the coating on DD floppies.
A drive that detects a DD disk will only write at the lower current standard for DD disk magnetic coating.
As a result,
a) the new written DD signal will be weaker than specified for HD medium
B) any data or format information previously written at HD current level will not be fully erased by a DD write operation
c) the ghost of previous HD data or format may last longer than the DD signals that are written over it.
Logically, if a more thoroughly magnetized HD "ghost" signal persists in the coating medium, it could over a period of time re-magnetize the weaker DD signal, the way any permanent magnet will do. In fact, this would happen even if the disk is bulk-erased or de-gaussed before a DD write, because whatever polarity is written as a particular DD signal point will eventually be affected by polarity of neighbouring molecules that did not take up the weaker DD charge.
I can see that this is a greater problem for diskettes than for CDs. CD/DVD signals are recorded as binary on/off optical bits, so there is quite a wide range of tolerance for stronger or weaker laser reads and writes. But floppies record a frequency modulated (FM) signal with a carrier frequency that is much higher than the data rate. This would be very prone to failure as the signal fades or degrades, as we know from floppies in long-term storage.
5.25" floppies retain data generally longer than 3.5" floppies - I have some that are 35 years old and still can be read.
I've ordered some DD disks.
Rick