I believe WD will wipe the drive anyhow themselves too. And I kind of doubt they'd care what you have on the drive, if they even look. I think they have much better things to do then look at all the contents of your drive. Unlike the Geeksquad people who have nothing better to do
But yeah, doing a "full" format using Windows should be good enough. It'll make it a timely process to recover anything. Otherwise use some of the other tools suggested which 0 out the drive then write random data then 0 it out again.