It depends on how paranoid you wish to be or have reason to be. There are a whole stack of utilities out there (some free, some not) that will overwrite all sectors with zeros or whatever many times to US security standards. Even so, it is just possible that a professional security expert with special equipment might be able to extract a few scraps. The average joe (or even computer guru) won't.
There was a PPRuNe thread about this a while ago if you can find it. If you do have really really terrible secrets to protect then your company will have security policies that normally destroy drives and melt down the bits.