What causes 'bad sectors' on a hard drive?

Well sorry to say :

Utter rubbish hard drive i will never support it again because they do not honor their warranty. I have owned 640 GB 2.5" & 1 TB 2.5" and looked after them properly but they ended up having bad sectors. I bought my drives from 2 different locations of the world,USA & Philippines.Both looked after very well, but i ended up having issues with Cyclic Redundancy Check.Lifeguard Tools reported bad sectors and it suggested that it may be able to fix the errors but it was unsuccessful. sorry its junk.

I just can not explain how frsutrated i am being passed along to phone other distributers here and they can not help me because i bought it over seas. its really ridiculous and i gave Western Diginal Elements a second chance which has caused me more pain.

If Western Digital can not help me then i am not prepared to support the brand and i will do the right thing and not recommened i