Create OS X Yosemite Boot Disk

$ /Applications/Install\ OS\ X\ Yosemite\ Developer\ \
  --volume /Volumes/Untitled --applicationpath \
  "/Applications/Install OS X Yosemite Developer"

Set value of --volume to your USB drive’s mount point

To determine drive mount point:

$ df -lH | grep "Filesystem"; df -lH | grep "/Volumes/*"

warning: the last stage of the Yosemite install can take a long time to complete. I have seen this on several machines. Let it run - it does finish eventually.