At first, find the disk corresponding to your Pi’s SD card:
Open Terminal, run diskutil list:

$ diskutil list
/dev/disk0
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *320.1 GB   disk0
   1:                        EFI EFI                     209.7 MB   disk0s1
   2:                  Apple_HFS Macintosh HD            319.2 GB   disk0s2
   3:                 Apple_Boot Recovery HD             650.0 MB   disk0s3
/dev/disk1
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:                                                   *0 B        disk1
/dev/disk2
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:     FDisk_partition_scheme                        *2.0 GB     disk2
   1:             Windows_FAT_32 boot                    58.7 MB    disk2s1
   2:                      Linux                         1.9 GB     disk2s2

Clearly /dev/disk2 is my 2GB SD card, the Linux partition name is also a bit of a clue.
However, instead of using /dev/disk2 with dd, you should use /dev/rdisk2

On the Mac you don’t want to be using /dev/diskn, you should use /dev/rdiskn instead, where n is the number the OS uses to identify your SD card. This decreases the time required to copy by a huge amount. Continue reading Backup and restore RPi SDCard on MAC OS →