Writing an image to a USB drive is somewhat convoluted in OS X...

write-image.sh

# Insert drive.
# See what drives are available.
diskutil list

# Unmount the target drive.
diskutil unmountDisk /dev/disk1

# OPTIONAL step to convert ISO to usb compatible image (ubuntu ISOs require this)
hdiutil convert -format UDRW -o ~/path/to/target.img ~/path/to/ubuntu.iso

# Write the image using dd(1). Experiment with blocksize (bs).
# Note usage of *r*disk to improve write speed.
# MAKE SURE YOU HAVE THE CORRECT DRIVE
sudo dd of=/dev/rdisk1 if=loonix.iso bs=8m

# Press ^T (Ctrl-T) to see progress while copying.

# Eject disk afterwards.
diskutil eject /dev/disk1

If you are writing an OS image and want to test that it is bootable after writing, try qemu:

sudo qemu-system-x86_64 -m 1024 -hdb /dev/disk2