Most Amiga floppies (pre-1200 and 4000) are DD. If you're trying to reformat a HD disk with the old DD standard, you need to cover the HD hole.
I'm not sure I've ever encountered a material that can do this reliably, short of epoxy. But for sneakernetting you can hack something up, or use a slip of index card or similar to ensure the switch in the PC's HD floppy drive is held down.
