Q: cant download mountain lion.. "osx cannot be installed you have s.m.a.r.t. errors"

Quick question, did you do what it said? Did you back up your files?

Back up your system ASAP and have the drive replaced.

Your hard is failing. http://en.wikipedia.org/wiki/S.M.A.R.T.

You should always be doing regular backups.  Disks can fail at any time; so can other components; directories can get corrupted beyond repair, users can make mistakes, etc.

You might want to review the Time Machine Tutorial, and perhaps browse Time Machine - Frequently Asked Questions.

If you don't like Time Machine, see Time Machine - Frequently Asked Question #27 for some alternatives.

Did you purchase AppleCare with your Mac?  If so, the drive will be replaced at no charge.

My Mac isn't even 12 months old and this has happened... Every other thing is fine... Everything saves and is retrievable from my hard-drive...

TheProdigy961 wrote:

 

My Mac isn't even 12 months old and this has happened... Every other thing is fine... Everything saves and is retrievable from my hard-drive...

The S.M.A.R.T. system is predictive in nature. It tries to warn you before the drive fails completely. Ignore it at your peril. Make sure you have current backups every day.

http://en.wikipedia.org/wiki/SMART_criteria