The Headers are deleted from the HDD, but the files themselves are not they just do not appear any longer on the HDD Map.
Data Recovery Software particularly Forensic Data Recovery Software can recover files because they do not look at the Drive Map but at the actual entire HDD itself and read fragments or more from the HDD.
Also as you can never completely erase Data off a Magnetic Device the Forensic Recovery Tools look for images which are not quite right and are slightly out of Sync with the current Data on the HDD. Though here by saying you can never completely destroy Data in no way implies that it is easy to recover or can be done cheaply. If you are willing to spend the money most things can be recovered but not many people I know of are willing to spend the $1M + per drive involved for a Full Forensic Recovery. The only places that I know who do this are the Law Enforcement Authorities for legal Cases.
The only sure way to destroy Data on a HDD is to remove the Magnetic Covering on the Platters in the HDD which effectively means destroying the HDD.
If all you want to do is remove the data from the current Partition Drive Map use the Shift Delete Process but if you want to permanently remove the data destroy the drive.
Also as this is a question you should post is as a question not a Discussion, the people at TR who answer questions do not look on the discussion threads all that often so you are missing the majority of those who will attempt to help you.
If you want a full description of how Forensic Recovery Tools work there is a lot of Mathematics involved and you need a In Depth Understanding of how a HDD stores Data to understand the full explanation. ;)
Here is a simple explanation of how a HDD works though it is a bit dated
http://www.ehow.com/how-does_4565085_hard-drive-store-information.html
How forensic Recovery Tools work is far more complicated and a lot harder to understand.
Col
