I found something new on this case.
Extra information: I am formating the SD card in FAT16 bootable.
I found that once the CHDK fails to save an image (ie. the image is filled with zeros) then even if I start the camera in normal mode (i.e card unlock.. regular camera firmware) the original camera firmware will also fail to save on the same way, ie. it will create zero filled files.
Any hint on this problem?
Could it be a relation between this camera, CHDK and FAT16 ? .. or even this SD card that is not ok?
I couldn't find however any pattern how to reproduce it.