There is no way to keep the data from ability to of being partially retrieved..
there is still always a way to do surface scanning that can retrieve data.
there are only ways to make it more difficult, or more hard, lesser data to be retrieved...
nothing more..
best way is zero fill , if you do this several times u make it is almost impossible to retrieve any data...
also a good way is to bottleneck the hard disk many times with rubbish data...like copying a folder so many times, and then recopy the main folders...over to keep the hard disk all data written all over again.
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