参照http://shumeipai.nxez.com/2014/05/18/raspberry-pi-under-mac-osx-to-install-raspbian-system.html

（/dev/disk1s1是分区，/dev/disk1是块设备，/dev/rdisk1是原始字符设备）

mac下在将镜像写入sd卡中出现如下情况dd: invalid number ‘4m‘ error，

解决方法来自http://www.raspberrypi.org/documentation/installation/installing-images/mac.md

• e.g. sudo dd bs=1m if=2014-09-09-wheezy-raspbian.img of=/dev/disk4

  This may result in an dd: invalid number ‘1m‘ error if you have GNU coreutils installed. In that case you need to use 1M:

  sudo dd bs=1M if=image.img of=/dev/DISK

  This will take a few minutes.

MAC下安装树莓派的镜像到SD卡中