this may possibly indicate a bigger problem
Western Digital RE4-GP 2TB Drive Problems | Pieter Viljoen's Blog
I seem to remember those early green 2TB drives had other issues as well....can't remember the details, but I have vague recollections of a firmware flash being required
could be worth looking on the WD site to see if there are any updates there.
Having said that, trusting a drive over three years old with key data is iffy