Thanks for the added info.
1. The only devices "added" - and used wirelessly - are a bluetooth Mighty Mouse, Time Capsule (for internet Wi-Fi), and HP Printer. All of these work as expected with Mavericks and Yosemite. I use an EXHD Firewire/Thunderbolt for doing Time Machine backups because to do so wirelessly with the Time Capsule base station is painfully slow. The EXHD is only attached to the MBPro when I manually want to backup once at the end of each day; otherwise it is unplugged.
2. How would I go about determining if there is some problematic KERNAL EXTENSION?
3. The MBPro has a built-in SUPERDRIVE which has been discontinued in recent models of this computer. Is it possible that Yosemite is "getting confused" by its presence in the machine?
Note: I had thought that perhaps finding the Mighty Mouse during startup was causing the freeze, so I tried a cold start without the mouse turned on. The computer still froze - and the screen went white - before either the keyboard lit up or the trackpad were activated. That is to say, when the Apple Logo disappeared and the screen got stuck, the keyboard and trackpad did not function . . . so had to force shutdown by pressing the power button; then pressing it again to start up.
