This article is from the Apple II Csa2 FAQ, by Jeff Hurlburt with numerous contributions by others.
Probably, the drive's Write signal is ON when it shouldn't be. This happens with some 5.25" drives when a 74LS125 IC on the drive's circuit board is blown. (I accidentally offset a row of pins on the //e disk controller card when plugging in a drive and blew the IC.) I removed the drive cover and replaced the 74LS125-- it was socketed-- voila! it worked again. (For possible disk-trashing fix for DuoDisk drives, see Q&A 006.)