Previously I could change the boot volume in System Preferences or do an "Option Boot" until I created the second partition on the drive and installed Yosemite on it. Since then, no matter what I do the only way to boot from either partition on the external drive is to use the Option boot from a COLD start.
Your comments and observations are always welcome. It is somewhat reassuring to hear that it might be a bug.
When you say "COLD start", do you mean from a shutdown as opposed to a restart?
