Creating a bootable USB or SD on OSX

First, verify the path to your SD card. You can do this by running the following command from terminal: 

diskutil list

The output shows a list of disks currently mounted on the system. Here's the relevant line from my output: 

/dev/disk3
#:                       TYPE NAME                    SIZE       IDENTIFIER
0:     FDisk_partition_scheme                        *8.0 GB     disk3
1:                 DOS_FAT_32 RPISDCARD               8.0 GB     disk3s1

In this case I can verify /dev/disk3 is my SD card because the TYPE, NAME and SIZE values are correct. 

If you have an existing partition on the disk you may need to unmount it, otherwise you'll get a "Resource busy" error message when you try to write the image. 

diskutil unmount /dev/disk3s1

Now to write the image file to the disk. Note the 'r' added to rdisk3 which drastically improves write performance by telling dd to operate in raw disk mode:

sudo dd if=RetroPieImage_ver2.3.img of=/dev/rdisk3 bs=1m

Depending on the size of your SDcard this may take a while. You can press CTRL+T to see the current status of dd. As an example for my Samsung 8GB SD card the write speed was 12MB/s the command took 11mins to complete.