If you are have a card and you want to make absolutely sure its empty you should overwrite it with data you dont mind sharing then reformat it. There really is no way to completely erase a card that I know of ; but by overwriting it all that could be recovered is the data you put on it that you dont mind sharing.
I recently loaned some cards out and wanted to make sure they were absolutely empty when I got them back.
Here are the three steps I took.
1: I put as much music on the card as I could.
(I have friends you use huge jpeg images of landscapes for this)
2: I placed the card in a camera pointed it at the wall and shot video until it said the card was full.
3: I reformatted the cards.
Some images and data could be recovered after a card is erased or the files are deleted.
It is less likely that data could be recovered after formatting a card but possible.
Either of these would take special software, but it is available.
