Storage not able to write, but Flash can?

at least P8 watch has SPI shared with display, didn't you try writing to display? also the flash chip can be sleeping, you can try something like

FCS=D5;
var fc=new SPI();fc.setup({sck:D2,miso:D4,mosi:D3,mode:0});
fc.send([0xab],FCS);//wake flash from deep sleep
fc.send([0x90,0,0,1,0,0],FCS) // get flash id
fc.send([0x9f,0,0,0],FCS); //get flash id

hopefully it should print something else than all zeroes or all FFs