Programa C no se ejecuta correctamente

[sam90]

Leiste la descripcion de los modos para abrir un archivo??

"w": Create an empty file for writing. If a file with the same name already exists its content is erased and the file is treated as a new empty file.
"a" Append to a file. Writing operations append data at the end of the file. The file is created if it does not exist.

Con "W" creas un archivo nuevo, borrando el otro. Con "a" lo abris para agregar al final. Saludos