update OSX Yosemite & cksum mismatch
You might have invalid Yosemite Installer package.

Try the Command+Option+R during boot to recover it from Internet.

To check the flie run following in the Terminal

shasum /Applications/Install\ OS\ X\ Yosemite.app/Contents/SharedSupport/InstallESD.dmg


The result should be:

eebf02a20ac27665a966957eec6f5e6fe3228a19  /Applications/Install OS X Yosemite.app/Contents/SharedSupport/InstallESD.dmg


Do this before running the Install.