[Techtalk] bad blocks

[ed orphan]

Can anyone tell me why a hard drive develops bad blocks?
I read somewhere that a good file system will detect and
isolate bad hard drive blocks so that they are not a problem.
Is this true for the ext3 file system?
And how many bad blocks can you expect before the
hard drive is unstable?  I've got an 80 Gig hard drive with
bad blocks on a 2 Gig partition. The drive is only two years
old! I've had drives for 6 - 7 years that are still clean ( well,
they do make a lot of noise these days ) and still work fine.