I haven't been here for quite awhile. But I came across your post when I visit here today and I hope my response here is still useful to your situation and not too late.
I understand using Format will not fully erase your data and a third party recovery program can recover some of the data in the disk. I'm not sure how accurate this info is. But the safest and sure way to delete data completely is to use a third party program ( the military also use a similar one ). I've been using this for years, and is also a free program. Have a look here
It may take a few hours to completely clean out your hard disk, depending on how big is your drive. But good luck and I hope this is still useful to you.