Ira, I don't want to hijack your thread, but I am having a similar, possibly related problem, but no solution. It sounds enough like your problem that I thought I would share it in hopes it might shed some additional light.
I have a copy of the Yosemite public beta installed in a partition on an external USB3 drive, there is a bootable copy of Mavericks on the other partition. 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. Even when the boot partition is nominally Yosemite even a restart from within Yosemite ends up booting from Mavericks on the internal disk. I have reported this to Apple as a Yosemite beta issue, but from your report it sounds as if it might be a prior issue.
as an aside other than the boot issue the Yosemite beta is working well and the only failure I have encountered so far is the drivers for a Dymo Label printer and Dymo reports they are working on that. The new iCloud features are interesting, and according to Apple what you place on iCloud will remain there, but access to is a sometimes thing as they are still working on the iCloud software.
