It’s inside your computer, and it doesn’t care whether you are a gaming maniac or an internet a holic. As long as you have to turn on your computer daily, one day it will make Murphy’s Law apply to you too: bad stuff will happen when it can cause the most damage.
Take a look around the internet; you’ll see you are not the only one concerned about how long your data will live. The crude fact is: your drive is not immortal. For most people who uses their computer daily, their HDD (from this point, it means magnetic platter drives)’s life will span from 3 to 5 years , really short if you ask me. If you don’t have enough money to replace a drive every time this period comes around, you’d better not store anything important there.
Besides obvious factors that will damage your data like a magnetic field, electric shock or physical shock, an invisible element could be temperature . There are HDD fans to address this issue, those fans are quite cheap, they cost from $2 to $5. Others claim those fans will blow dust to other components. Well, if they would mitigate the heat issue that would not be a problem with me, since I take my time to physically clean my computer’s components every 3 months.
From the above source , there is also a ridiculous sounding argument: spinning drives will cause drive motor’s lubricant’s to spit out and condensed on the platters, making the drive crash. If that were such the case, why would they use such a lubricant in the first place? However, the spin may have something to do with it, as with every motor, any kind of movement would wear them out over time. It’s possible to stop your drive from spinning when you don’t use it through power management or some utilities. You can’t use this however, if your drive is frequently read from or written to .
Years ago, some guy from Quantum claimed that there’s little can you do to prolong your hard drive’s life span , so does Google’s 2007 research 
Failure rates are known to be highly correlated with drive models, manufacturers and vintages. Our results do not contradict this fact For example, Figure 2 changes significantly when e normalize failure rates per each drive model Most age-related results are impacted by drive vintages. However, in this paper, we do not show a breakdown of drives per manufacturer, model, or vintage due to the proprietary nature of these data.
You may think that high usage will make your drive fail you sooner. Actually the situation is more complex than that.
Hard drives less than three years old and used a lot are less likely to fail than similarly aged hard drives that are used infrequently, according to the report.
“One possible explanation for this behavior is the survival of the fittest theory,” said the authors, speculating that drives which failed early on in their lifetime had been removed from the overall sample leaving only the older, more robust units.” 
So, use your drive frequently when it’s still have warranty to make sure they are good, before giving them a rest :P. Some may say drives nowadays’ lifespan is shorter than before because of their high spin rates. That may be true though, but I can recall I hardly have my hard drive read or written when I was programming Pascal on a ram drive back in my 486 time :). As a matter of fact, the two hard drives I have back then still work now, a 200MB and a 800MB with spin rate somewhere below 5k. Sadly, you can’t have Visual Studio 200x to compile anything without writing to your HDD, even if you lay all your files on memory, VS would never fit csc there.
Back to the main topic, Google’s report also reveals that the temperature – failure correlation is complex too.
As you can see, the lowest failure rate is reached when the drive is around 40 degrees Celsius, be careful applying HDD fans. Though I think I will need one when my drive get older. When transferring the aftermath remains from the crashed drive to the new drive, I noticed the older drive (Samsung) is hotter than the newer (Seagate), though I’m unsure this is because of their age or being a brand matter like Google said.
Even though they say SMART failure somehow correlated to actual drive failure, you can’t depend on that not-so-smart feature to know when your drive is going to fail you. Actually, Google can’t even model how SMART errors affect failure rates
The attribute with highest failing rate is “anything” :P.
Google’s research shows that drives which have their first bad sector have 10 times probability to fail that intact drives. I may agree with this. The first time you hear a click from your drive (I mean a clear click you can hear from half a meter away, from outside the case), you can start backup your data, as your drive may fail you anytime from the next 2 weeks.
In the end, what you can do, you don’t know (because Google is mean :()! Despite being extensive and well conducted, Google’s research can’t accommodate every possible use case a normal user (e.g. you) will encounter. You can’t just stop yourself from turning the computer on (to control drive usage) those days, the net has become something you need like air, water or food! Thus, it’s best to simply do what you feel best to keep what you value most with you. There is a tip that works for everyone though: backup early, backup often and make as many backups as possible.
For a conclusion, there is a Wikibook dedicated to this topic, which I would recommend. 
When netbooks comes out, a new kind of HDD is introduced for the public too: solid state drive or SSD (my definition for a hard drive is something fixed in your computer and is not a removable media drive). My first opinion about them is that they won’t last long, since their life depends on the number of times data is written on them, what would happen if you download stuff, short on memory and the OS have to swap, or surf the internet with them (in which case the cache will get written and rewritten, again and again). Some of my colleagues already have their pen drives (which is basically the same technology as SSDs) failed after about 2 or 3 years usage. And lastly, the netbooks are cheap, the pen drives are cheap to, and cheap stuff rarely had high quality. 😛
However, some folks from a Mac forum may have proved that I’m wrong . SSD can be written around 1 million times, plus they have 25% bonus capacity for wear leveling. They calculated that SSDs could outlive the owner if used sparsely. Well, time will tell.
References(Don’t flame me for not having them sorted)
 Why good hard drives go bad
 Failure trends in a large disk drive population
 Minimizing hard disk drive failure and data loss
 How to Take Care of Your Hard Drive to Prevent a Crash
 BAD SECTOR: bản chất, nguyên nhân và phòng tránh
 Using spin down to prolong the life of old hard disks
 Hard disk test ‘surprises’ Google
 Replace Hard Drive with Flash Card “SSD”