Open a terminal. BEFORE inserting the USB key, check which disks are available:

You should see one or more disks installed on your system like this:

Insert the USB key. Do a “diskutil list” again. You should see one more disk. This is your USB key. Let’s assume it is /dev/disk2First we need to convert the ISO file to an image using this (exemple here with Debian ISO):

This will create a debian-8.0.0-amd64-netinst.img.dmg file. Hdiutil adds this extension by default. Let’s remove it:

Now we are ready to write the image to the USB key:

And voila! Once flashed Mac OS will ask you if you want to eject it. Say yes. Your key is ready to use 🙂