You may be exercising excessive paranoia. On the one hand, if someone wanted to, they could extract data no matter how many times you write on the disk, as the heads are not always exactly in the same place, and some peripheral trace of magnetism may remain.

On the other hand, the technicians will probably fire up the drive, have a quick look for p0rn, and if there's nothing worth copying, chuck it in the skip before sending you a replacement.