Just to let all know, as reported on osxdaily, I had similar problem...a weekend nightmare of trying to install Yosemite. After 2 clean installs, I figured out Yosemite just doesn't work (get beach ball on restart/login) with BT Net Protect Plus (McAfee free for BT Broadband customers). I thought Yoesmite was a pile of c**p...turns out it's the mcAfee. Wish someone told us!. So, if you have installed Yosemite or are trying to, uninstall McAfree using the uninstaller and you will be fine. Horrible nightmare.