Installation of Yosemite taking over 20h
I'm not recommending this to anyone, just thought I'd report on what worked for me:


Let Yosemite upgrade sit at the white progress bar screen at 50% for 4 hours.
Once I was nearly positive it wasn't doing anything like writing to the hard drive...
Held power button for 10 seconds to turn computer off
Waited about 10 seconds again
Turned power on and held Command R  (It took about 3 minutes to boot)
Clicked on WI-Fi symbol on upper right and turned off Wi-Fi
Selected restart from the System menu option
Computer rebooted and finished the upgrade


I have since rebooted (twice to be sure) and run a few things and all seems fine at this point.