Hey Pierre - This is a common concern with the BB 9XXX series. Users have however reported to have been meet with success trying out different measures like replacing the SD Card or battery. Other possible causes could be:
- Dirty battery contacts - make sure both the contacts on the phone and those on the battery are free of carbon (you can use a cotton swap mildly dipped in methylated spirits to wipe both and let to dry)
- A system glitch - this may require you try different versions of 9720 firmware (starting with the latest available) to be sure is is not an issue that is firmware borne
- A defect main PCB flash light (or its dependency) component - this will require that you turn the 9720 in for service unfortunately as working with tiny and surface mount components requires special equipment.