This article is from the Apple II Csa2 FAQ, by Jeff Hurlburt with numerous contributions by others.
I recently ran into this problem with a Disk II. It insisted the disk was write protected. I checked the write protect switch, but it was okay. I pulled all the chips out of the analog board inside the drive, cleaned the contacts with a clean pencil eraser (one was really dirty, almost looked burnt) and re-installed them. Voila! The problem went away. By: Rubywand