Today I started the upgrade of my Mid 2011 iMac 27 from Mavericks to Yosemite. Initially it got stuck at the end of the first part of the installation. I could however pull up the installation log using Command-L and so I could tell it was still doing something. Eventually it completed and rebooted. It then got into the Completing Installation phase with a similar progress screen. It's been at "Less than a minute remaining" for the past 4 hours and no sign of activity. I also can't pull up the installation log by pressing Command-L. I just hear the familiar error sound when I try that.
I do use Homebrew so there's probably a lot of stuff in /usr/local causing the installation to take a long time, but I figured that's what caused part 1 of the installation to take so long, but it doesn't explain why part 2 is taking even longer.
Mavericks was installed on an external SSD (Lacie Rugged series) connected through thunderbolt. I now read that some people are experiencing problems with 3rd party SSD drives in Yosemite (yeah, I know I should have checked that before starting the upgrade, but who would have known?).
I'm not sure what to do now: just power off the computer and see if it continues finalizing the installation process after I restart it? Or just wait until tomorrow morning and see if it's still stuck? Even though I have a time machine backup on an external drive I don't want to risk losing my data forcing me to do a restore. Any advice?