Simple problem - dreadful solution.
Bought a Cubot J3 - admittedly a cheap end of the market model. I put some music files (mp3) on internal storage. They worked fine. But I decided they were taking up too much space and I would move them to the SD card. I deleted the files from internal storage. It did not free up the space. Contacted Cubot support. After some detailed conversations and screenshots they said the only solution was a factory reset. This, to simply delete files. I shall be buying a more reputable phone. Their response, like their product, is near useless. The offer of support in the reply turned out to be worse than the first solution they proffered. Support is truly dreadful.