If you use the recovery image to put it back in its original state, then you could use something like
http://eraser.heidi.ie/ to go over the empty portion of the HDD that should be good.
Are you sure the SSD contains the recovery image? Usually the SSDs are used to cache the HDD stuff to make the system faster, putting the recovery image (something that is never used and never changes) on the SSD seems unlikely. Or maybe the 32GB is just a separate partition and the SSD isn't actually visible from Windows?