27" imac yosemite update hangs. Required power cycle and safe boot to get the update going. Now all OS X update do the same.

27" iMac Yosemite update hangs.

The original upgrade to Yosemite (10.10) took almost a full day! The computer hung during one of the reboots and just showed a blank grey screen, no apple logo, no progress bar, nothing. I left it like that for about an hour thinking it might be working in the dark... Eventually I had to power cycle. Disconnected power at the wall outlet for about 5 minutes, then reconnected and tried to start. After about 20 minutes, got to the same point, grey screen:no action, again waited at least an hour.

Then I did a safe boot. System came up OK in safe mode to the log in screen. As soon as I logged in, it proceeded with the interrupted update, which ran to completion. Yay!

Now, every update to Yosemite that arrives gives men the same grief!

To add some more, the "Pro Video Formats 2.0.1" update has installed about 4 times, and is asking to be installed again? There's no indication that it fails the update so whats going on?