esp32 E.openFile stops working after multiple fs.readFile and fs.writeFile

Could it be that it interferes with the upload of the code? ...because it is executing as part of that task. In first shot, just remove or comment the line with writeData(0);, upload the code, and invoke the method from console. (I do not know your other settings... but in general, putting active code into the upload is not to recommend. Other settings in the IDE may make this an non-issue. Also that it is on a ESP32 chip may render my concern an non-issue, since most of my experience is with the Espruino boards...).