I was booting for the first time a Raspberry Pi with NOOBs, the installation was doing fine until it got stuck for more than 20 min. After a quick read on the internet it was apparently fine to power-off the raspberry.
I removed the SD card and put it in my MacBook to do a clean install, however I realised that the SD card wasn't appearing, either on disk utility or in the terminal. I tried to reboot my laptop, nothing changed.
Finally I tried to slide down the little switch on the side of my SD card adapter, and now I can see two partions : RECOVERY and boot. If I understood correctly my micro SD card is in read-only mode. How can I erase my card if it is only appearing in read-only?