SD Card: Read-Only file system
I had a similar problem after installing Windows 10 IoT for raspberry pi on an sd card and it managed to make the card and file have no +w on directory/file. This was mistake number one as it turns out me managing to write this to a card on a Mac was never going to work on the pi. So the problem remained of how to reformat, as erase in disk utility would not work and no use of chmod would change the bits. Tried to change on an ubuntu box with the same problem "Read-only file system".

The only (hacky) way I managed to do reformat the card was to put it in a camera and reformat there - and bingo - done.

I'm sure there is a legit way to do this (maybe on windoze but I don't have one of them), but the camera trick worked like a charm. It was a fujifilm x100s in case not all cameras can manage it.