Obtain the FreeNAS image. For this example I am using FreeNAS-amd64-embedded- Have a USB stick on hand that hopefully supports booting.

Insert the USB stick into the MAC USB port. Open a terminal to find out which device the USB stick is.

diskutil list

The output should be something like:

   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *500.1 GB   disk0
   1:                        EFI                         209.7 MB   disk0s1
   2:                  Apple_HFS Macintosh HD            280.1 GB   disk0s2
   3:                  Apple_HFS Data                    219.5 GB   disk0s3
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:     FDisk_partition_scheme                        *4.0 GB     disk2
   1:             Windows_FAT_32 KINGSTON                4.0 GB     disk2s1

Since I am using a Kingston USB sick “disk2” is the device. This translates to “/dev/disk2”. Next unmount the disk:

diskutil unmountDisk /dev/disk2

Then write the FreeNAS image to the USB stick:

gunzip -c FreeNAS-amd64-embedded- | dd of=/dev/disk2

This should result in output like this:

147168+0 records in
147168+0 records out
75350016 bytes transferred in 108.529822 secs (694279 bytes/sec)

Next eject the disk. This should ensure all data is written to the USB stick before we remove it.

diskutil eject /dev/disk2

Finally, remove the USB stick from the USB port.