I can't install Yosemite on my 500GB WD external drive. Download went fine, no problems with the code delivered by apple, launching the "install yosemite" from the launchpad, no problems, macbook restarts, installion continues after a few seconds, a screen pops up "Install Yosemite" and i can't do anything. Any clue? I am running mavericks 10.9.4 with no problem. Thanks for the answers.