A short clip:
3.5" DS2D and DSHD media are very close in terms of the magnetic properties of the coating. DS2D media generally is coated with an oxide coating with a coercivity of about 660 Oersteds (Oe); DSHD media uses a coating with coercivity of 720 Oe, so it's only a bit "stiffer" magnetically speaking. DSHD also generally uses a smaller particle size, which isn't bad for DS2D use.
So far, so good--it looks as if the HD media will serve passably even if the write current in double-density mode isn't quite as high (lower-coercivity = lower write current needed to "flip" the domains). By the same token, a 2D diskette should work passably in an HD application (bring up any memories of the "puncher" accessories that used to be popular?). [These punches were sold for people to convert then-popular DD diskettes into "looking" like then-rare and expensive HD diskettes, with mixed results. - Herb]
Where things start to get interesting is in the coating thickness. DS2D coats the cookie with a layer about 1.9 microns (um) thick. By comparison the average fine human hair is about 25-30 um thick. (Hair diameter depends on the person and the type of hair, the range is pretty wide; from about 12 to over 100 um for a single hair diameter).
However, the problem arises that DSHD media uses a coating about half that thickness (0.9 um). This is why the cookie in a DSHD 3.5" diskette seems almost transparent if you hold it up to a strong light, while the DS2D variety is opaque
