The Memotech MTX Series
Floppy Disk Media
The Oersted (Oe) is a measure of magnetic field strength and is used
to describe the magnetic properties of floppy disk media and its
coercivity - a
measure of the resistance of a ferromagnetic material to becoming
magnetized/demagnetized. Whilst 3.5" Double Density (DD) and High
Density (HD) have very similar magnetic properties, DD media has a
coercivity of about 660 Oe and HD media is about 720 Oe.
The coating on a DS diskette is about 1.9 microns thick, while on an
HD diskette, it is about 0.9 microns thick.
What this means in practice is that when a HD drive sees an HD
diskette, it knows to write data using a higher current than if it were
a DD disk. If you cover the hole in an HD diskette to trick the drive
into thinking that a DD disk has been inserted, it will write data at a
current level compatible with DD disks, i.e., LESS than is required to
securely write to HD media. While the disk may appear to be fine, it is
highly likely to lose data very quickly.