In the final analysis, Hard Drives are mechanical devices and therefore one of the most vulnerable component in a computer to failure. The logical and sensible thing to do with such a weak point is to have the ability to restore the infomation contained on the drive.
That is why I use Acronis True Image to create images of my Master drive. In the event of catastrophic failure of the primary master HDD, the drive content is completly restorable to the last backup image created and stored on the slave or external HDD. This should includes the OS, program applications and files along with whatever hidden partitions provided by your computer mfr that you desire to retain.
Certainly, there is always the remote possiblity of failure of both the primary master HDD and the primary slave HDD (or in the case of my laptop, the external enclosure with a WDE HDD inside)... But the simultanious failure of both is highly unlikely. One must play the odds to some degree. IMHO, Attempting to predict when or if a HDD will fail is an endevor in futility. This question requires a simple exercise in CYA.
Raybro