Is is possible to format and use 5.25 high density disks in a double density drive?
No. The coercivity of the media is different. HD disks are higher coercivity, so a normal DD drive would not have the requisite head strength.
Coercivity of floppy disk types:
|Size||Density||(Usual) Formated Capacity||Coercivity|
|8"||all formats||300 oersteds|
|5.25"||double density||360 KB||300 oersteds|
|5.25"||quad density||720 KB||300 oersteds|
|5.25"||high density||1.2 MB||600 oersteds|
|3.5"||double density||720 KB||665 oersteds|
|3.5"||high density||1.44 MB||720 oersteds|
(Table taken from the Wikipedia entry about Floppy Formats)
Coercivity, also called the magnetic coercivity, coercive field or coercive force, is a measure of the ability of a ferromagnetic material to withstand an external magnetic field without becoming demagnetized. Coercivity is usually measured in oersted or ampere/meter units.