Yosemite won't install due to a "S.M.A.R.T. drive problem that can't be repaired."

Question

After downloading Yosemite successfully, it won't install due to a "S.M.A.R.T. drive problem that can't be repaired."  It says to back up as much as possible and replace the drive.  Everything works flawlessly with Mavericks.  What gives??

Answer 1

If you are getting a S.M.A.R.T. error then you should be more worried about losing everything on your drive then about upgrading your Mac OS.  Regardless of if you are running Mac OS, Linux, or Windows, you should never just disregard such a drive error.  S.M.A.R.T. is designed to give you a warning that your drive is statistically having far more errors than normal and should be backed up immediately and typically replaced.
In your case, it's likely a coincidence that your drive is failing at the time you are attempting to upgrade to Yosemite.  You should stop what you are doing, backup your drive (if you don't already have the backups that you should prior to any OS upgrade), and replace the hard drive prior to moving forward.
To make it clear, your hard drive is likely going to fail soon and you may lose everything on it at any moment.  This failure has nothing to do with Yosemite or any Mac OS.  All hard drives fail at some point.  Hopefully this S.M.A.R.T. notice has given you the opportunity to solve a major problem before it gets worse.