I discovered the hard way why many 400K disks didn't work well as 800K disks: The 400K drives can destroy one of the surfaces. The business side has a head; the other side rests on a post. That post can be abrasive enough to kill that surface. Maybe a clean, new 400K drive wouldn't cause this problem, but how many of us have such a thing?

Years ago, I took a few fresh 400K disks out of a box and had no problem formatting them as 800K. However, if I first formatted their brethren in a 400K drive, the yield as 800K disks plummeted. I could actually see score marks where bits had been ground away.