• Also, using D11 for CS is a bit scary, because that's shared with the button on the Thingy?

    Yes it is. I just unsoldered the button to avoid conflicts.

    are you sure you haven't just wired it to the wrong pin on the SD card?

    I am. In addition, I tried to remove the SD card just to be 100% sure there isn't any electrical conflict.

    driving the SPI directly from prompt (ThingySpi.write(0xAA)) gives the exact waveform on the MOSI pin. Calling E.connectSDCard(ThingySpi, CS), E.unmountSD();, fs.appendFile(File,data) or even E.openFile(File) has no action on MOSI pin.

    MOSI=D4;//P0.04
    MISO=D3;//P0.03
    SCK=D2;//P0.02
    CS=D11;//P0.11
    
    var ThingySpi = new SPI();
    var csvLogFile="logFile.csv";
    
    function Init()
    {
      // Wire up up MOSI, MISO, SCK and CS pins (along with 3.3v and GND)
      ThingySpi.setup({mosi:MOSI, miso:MISO, sck:SCK});
      
      E.connectSDCard(ThingySpi, CS);
    }
    
    
    function csvLogSD(s)
    {
      try{
        if(fs.appendFile(csvLogFile,s)==true)
          console.log("log saved");
      }catch(err){
        console.log("error writing to SD.. trying unmount+remount");
        E.unmountSD();
        E.connectSDCard(ThingySpi, CS);
      }
    }
    
    
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