My Lumia 640 LTE keeps informing me that my internal memory is almost full and that I must delete at least 100 Mb in order to allow certain apps to function. (For example, WhatsApp has stopped functioning.) However, all of the files or folders that I can examine using File Explorer are very small. There obviously must be almost 16 Gb of hidden data. How can I view and manipulate the hidden data? I would like to move as much as possible to the added 16 Gb flash memory card, which is almost empty.