Boot & Nuke
http://dban.sourceforge.net/ - offers DoD level of securely wiping disks.
I researched this extensively some time back and hope I'm remembering correctly... But, it's my understanding that reformatting will NOT clear all the data. It will look cleared in windows, put some/much of the data is still there - just not mapped. Supposedly pretty easy to access the data from a UNIX OS.
The only way to truly "erase" is to write over each partition,
repeatedly, with new meaningless data. Reformatting, nor deleting, just doesn't do that.
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