In one week of heavily usage I've seen that this error occurs with some apps or options that do use the sd card. Two examples given: 1 - Try to exchange some message with whatsapp, this app isn't stored on the sd, you can't do it, but it saves a backup file on it. 2 - Try to set as background image and/or lockscreen image some image that you have on sd card and not bundles images on the phone. In these cases I can get sda issue several times during a day of use. Let me know if you spot something following these steps.