@vkc If you use a HD floppy in a DD drive with the hole covered, you will run into problems eventually. The actual coating on the cookie is different, and the magnetic flux isn't as strong.

This will "work" usually, but you risk data loss very quickly - and this method should not be used if you expect a need to retain data on the disk.

http://www.brutman.com/Diskettes/Diskette_handling.html