Almost all Mac users used to save their important data and documents in Mac hard drive or in external HDD (Very Few). Saving data in external HDD is as important as you save your money in bank for security purpose, so in future you can use it. There is no doubt that data is more important than money today and lots of people don’t even care about it. Data protection is not a rocket science you just need to take backup of your important data via cloning, imaging or simply copying data into the external hard drive.
I have a Mac Mini OS X 10.7.2 at home and take regular backups on an external hard drive. My brother who also shares the same system doesn't care about taking proper backups. He believes that Mac is a very robust and secure operating system so there is no need to clone.
However, one day while he was doing some office work, the Mac got crashed suddenly. He tried to start the Mac but it failed to boot. When I returned home from vacation and got to know about the system crashed, I desperately tried to switch on the system but all my efforts went in vain. Hard drive was making a tickling sound and this sound shows hard drive is physically damaged and inaccessible. So I told my brother not to switch on Mac now as it can do more damage to HDD.
I could have lost all the data if I haven't created clone of Mac in the external HDD. I bought a new HDD for my Mac and after configuring it I started recovering data from the clone of previous hard drive. Thereafter recovering data, my brother was very much happy to see all his office documents back and now he understood the reason behind creating clone of hard drive.
Clone should preferably be created in the different hard drive or an external hard drive, so that it can be used in the event of an inaccessible Mac. There are some very effective cloning software available in the market. But I used Stellar drive clone as I found it cheap and reliable. To get information about the cloning software you can go through the review sites like http://www.toptenreviews.com/ , http://www.cnet.com/, etc.