One does not simply unmount a USB flash device with the OS X "eject" feature. Instead you'll need to run an OS X unmount command to put the device in a state ready for DD.
Firstly find our the disk identifier of your flash memory.
My disk identifier was
/dev/disk1 so lets unmount
/dev/disk1 with OS X's
disktil command and finally run DD.
diskutil unmountDisk /dev/disk1 sudo dd if=some.iso of=/dev/disk1 bs=8192
Obviously replace some.iso with the name/path of your image file.
Please Note: DD returns no feedback during the copying process and an entire ISO can take a few minutes so just sit tight and be patient!
Finally eject your disk.
diskutil unmountDisk /dev/disk1