just to confirm :

It only happens to the SLAVE HDD ??

is it after the 'same' amount of time that the HDD dies ?
Is the Master HDD OK ? STILL ?
Do you place the slave in the same cage as the master ? [that is, it isn't just 'dropped' into the case]
Is the slave fixed as per the Master ?

I'm thinking along the lines of static / power surge .................
Is the power cord Earth wire 'good' ? can you prove the continuity ?
No Earth, and the slave in a different set up to the master, can cause problems, diferent probs in diferent areas .............