The following is from an article on apple forums.
Hard Drive Warning (all makes and models)
Ironically but logical, new hard drives are far more fragile than one that has been working for several months or a couple years. So beware in your thinking that a new hard drive translates into “extremely reliable”!
Hard drives suffer from high rates of what has been termed “infant mortality”. Essentially this means new drives have their highest likelihood of failing in the first few months of usage. This is because of very minor manufacturing defects or HD platter balancing, or head and armature geometry being less than perfect; and this is not immediately obvious and can quickly manifest itself once the drive is put to work.
Hard drives that survive the first few months of use without failing are likely to remain healthy for a number of years.
Generally HD are highly prone to death or corruption for a few months, then work fine for a few years, then spike in mortality starting at 3-4 years and certainly should be considered end-of-life at 5-7+ years even if still working well. Drives written to once and stored away have the highest risk of data corruption due to not being read/written to on a regular basis. Rotate older working HD into low-risk use.
The implication of this is that you should not trust a new hard drive completely (really never completely!) until it has been working perfectly for several months.
Given the second law of thermodynamics, any and all current mfg. HD will, under perfect storage conditions tend themselves to depolarization and a point will be reached, even if the HD mechanism is perfect, that the ferromagnetic read/write surface of the platter inside the HD will entropy to the point of no viable return for data extraction. HD life varies, but barring mechanical failure, 3-8 years typically.
Hard drive failure and handling
The air cushion of air between the platter surface and the head is microscopic, as small as 3 nanometers, meaning bumps, jarring while in operation can cause head crash, scraping off magnetic particles causing internal havoc to the write surface and throwing particles thru the hard drive.
Hard drives are fragile in general, regardless, … in specific while running hard drives are extremely fragile.
PDF: Bare hard drive handling generic instructions
hard drive moving parts
Some of the common reasons for hard drives to fail:
Infant mortality (due to mfg. defect / build tolerances)
Bad parking (head impact)
Sudden impact (hard drive jarred during operation, heads can bounce)
Electrical surge (fries the controller board, possibly also causing heads to write the wrong data)
Bearing / Motor failure (spindle bearings or motors wear during any and all use, eventually leading to HD failure)
Board failure (controller board failure on bottom of HD)
Bad Sectors (magnetic areas of the platter may become faulty)
General hard drive failure
