I realised a few years back that most cheap floppy drives on PCs these days don't even have the DD/HD hole detection switch! They can't read or write DD disks at all because of this. I had to replace the floppy drive in my PC with an older one in order to transfer things to and from my Amiga.
I've always used Sony HD floppies on my Amiga, and they seem to work fine. Apparently, there is some overlap in the specification in the range of field strength required for HD/DD, and Sony disks tend to be in the overlap. I can't remember where I read that now.
