Q: El Capitan won't install after admin password entered

I have a MacBook Pro with two internal hard drives, both of which have ample empty space.  One drive boots into Snow Leopard (10.6.8) and the other boots into Yosemite (10.10.5).  My plan is to install El Capitan on the current Yosemite drive.

While booted in Yosemite, the installer leads me through the process up to the point where it asks me for my username and administrator password.  I enter these, press install, and nothing happens.  I remain in the “OSX will be installed on the disk” window.

I know I am entering the correct password because the icon does not “shake its head.”  It does shake if I intentionally enter an incorrect password.

I booted into Yosemite in safe mode, same behavior.

All this also occurs if I boot from my Carbon Copy Cloner bootable backup of the Yosemite drive.

Now, a couple other odd things have recently started when I boot from the Yosemite drive.  Many of the standard Apple apps will not launch (Mail, Preview, Photos, App Store, etc).  Doing a search within a Yosemite Finder window does nothing.

Any ideas?