If you are using an SSD with trim enabled Yosemite is a disaster. I’m still a Mavericks guy.
Had clone SN partition on ext HD which I attempted to upgrade variously from Mavericks (upgraded from Lion, which was upgraded from the clone SN) and directly from SN. All failures. AAPL forum deleted my post asking for help (it was in the correct forum and subject) - other posters with similar problems went unanswered.
And I'm still a Mavericks girl...actually old lady :-)