Jasons-MacBook-Pro:~ root# dd if=/Users/Jason/Downloads/ubuntu-14.04.2-server-amd64.iso of=/dev/disk3 bs=1m
dd: /dev/disk3: Permission denied
Jasons-MacBook-Pro:~ root# dd if=/Users/Jason/Downloads/ubuntu-14.04.2-server-amd64.iso of=/dev/disk2s1 bs=1m
dd: /dev/disk2s1: Operation not supported

Jasons-MacBook-Pro:~ root# diskutil list
/dev/disk0
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *251.0 GB   disk0
   1:                        EFI EFI                     209.7 MB   disk0s1
   2:          Apple_CoreStorage                         250.1 GB   disk0s2
   3:                 Apple_Boot Recovery HD             650.0 MB   disk0s3
/dev/disk1
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:                  Apple_HFS Macintosh HD           *248.4 GB   disk1
                                 Logical Volume on disk0s2
                                 AD8D7EFC-308C-492C-9354-BEAAEC1C6F01
                                 Unencrypted
/dev/disk3
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:                            U3 System              *100.7 MB   disk3