Doing device level copies with dd is a reasonably common Linux task, but not something OS X users typically do. However, if you find yourself needing to write an image file1 to a microSD card or some other media then here's a simple guide to using it.

First, you'll want to see the device labels.

diskutil list
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *250.1 GB   disk0
   1:                        EFI                         209.7 MB   disk0s1
   2:                  Apple_HFS Macintosh HD            249.7 GB   disk0s2
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:     FDisk_partition_scheme                        *2.0 GB     disk2
   1:                 DOS_FAT_32 THUMB_DRIVE             2.0 GB     disk2s1

The output from above shows that we have a mounted 2GB USB flash drive named "THUMB_DRIVE" that has been given the identifier /dev/disk2. To write to it we need to unmount it, but ejecting it in the normal Mac way won't work here.

diskutil unmountDisk /dev/disk2

There we go! Now we just need to write our image file.

dd if=inputfile.img of=/dev/disk2

The shell will appear to hang, but you can check disk activity to see that it's writing. Once the command returns you're done!

  1. Frequently these guides will talk about using Win32DiskImager on Windows