chrols

Just in case anyone is having this problem out of-the-blue, I was experiencing it and just before I was about to format the card, I checked the little lock switch on the card. It had somehow moved slightly from its unlocked position. Turns out, that was what causing it. I'm assuming it moved during the multiple times I would fumble plugging it into my Mac. Hope that helps somebody

Just found this post. I'm seeing this same thing here occasionally. I just got my UM2 yesterday. Running firmware from Cura 15.04.3.