i see, all your replacement drives are fine..
did you buy all drives from the same source? it might just be improper handling done by your source.
lastly..
do you move the drive while it's running?
lastly..
is there any magnetic/electro-magnetic device closer to drives when they are in operating condition ?
these can cause serious damage without you knowing..
a small device like cell phone closer to those hdds lying outside is good enough to form electromagnetic disturbances.