Hint:
read this page:
http://en.wikipedia.org/wiki/Floppy_diskSearch for "stronger" and you'll find the tidbit about magnetic head strength.
As for using HD floppies in DD drives, I also _never_ had any problems on Amigas. And afterall: you have nothing to lose. If one doesn't work, just trash it. There is an overabundance of HD floppies today ( although not in a few years :-) )
Cheers!
