Yes indeed, the Core2 is really terrible when it comes to flashing/burning images. I have a M5Paper that I use from the very same computer and have no problem at all flashing anything, but Core2 is very, very, very tedious.
I will take you many retries to finally succeed getting something burned. What I found is that sometimes pressing the reset button right after starting to burn helps. Might be just a mental illusion, but I have the feeling my success rate improved by 1 or 2%.
This implies you can burn an image to the core2 are you using windows or Mac OS? It seems there are major issues with the core2 which are not being addressed.