This article is from the 3b1 computers FAQ, by John B. Bunch with numerous contributions by others.
Jon H. LaBadie, Boyd Ostroff, and Perry Miller have suggested that there could be accumulated dust blocking the write protect apparatus on the floppy drive. The easiest solution for this is to get a can of compressed air, or blow real hard into the floppy aperture several times. This generally will remove the dust or dirt from the area, and hopefully will solve the problem. (With the light being blocked, the floppy drive is considered permanently write protected!)
If the problem persists after you follow the simple solution, you might have to open the machine, remove the floppy drive, and perform a closer examination. If all else fails, a standard TEAC 360K floppy drive is a one-for-one replacement.
 
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