If you're trying to use the dd command to image a usb disk or another device and you're running into an error that looks like:
# dd: /dev/disk#: Resource busy
There is a simple solution.
Use OSX's Disk Utility and unmount any of the partitions you have mounted on that particular disk without unmounting or ejecting the disk itself.
Afterwards, attempt the dd command again.
Attempting to use the umount utility in Mac OSX will result in a "Resource busy -- try 'diskutil unmount'". The command-line equivalent would be:
# sudo diskutil unmount /Volumes/<disk in question>
# sudo diskutil unmount /Volumes/FLASHUSB