Hammer the drive will effectively bend the platters and make the drive un-readable. If your clients are happy with this solution, then it's very time effective... Also, as long as your recycling buyer buys them like this... as, dirty aluminum maybe? Or just shred? I don't know. Taking them apart to separate out the parts after bending them would be a pain...

My clients ask for more and that's what I give them. You have to find out what will be acceptable and go from there.