how do HDD fail by age

Depends. Sometimes, the bearings stop working properly which will make it unable to spin.

Sometimes something on the drive PCB fails, or gets damaged.

If it's dropped, the head can bounce onto the platter, and gouge it, destroy the head, and leave a bunch of dust flying around.

Sometimes it's just software confusion, and re-formatting will fix it.
 
HDDs are electro-mechanical assemblies. All such devices will fail sooner or later due to mechanical wear on the mechanisms inside. That's why such devices have an "MTBF" (Mean Time Between Failures) to give the consumer some indication of the life expectancy.

All products, cars are a good example, will fail due to wear and tear.
 
As mentioned in the past 2 posts many things can cause hard drives to fail. I used to have a 5 year rule of thumb on consumer hard drives. If you get 5 years out if it you got your moneys worth and should replace it. Nowadays thats a bit short I feel that reliability has gone up. But keep in mind aside from fans (that are cheap and easy to change) and DVD players (who cares), its the only mechanical part in your PC, and all that movement can often cause failure.
 


For the right amount of money anything is fixable. But if you have to pay a service for data recovery on a drive can cost you somewhere around $1000 (which is why backups are important). And just physically fixing it to make it work isn't worth it.
 

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