WiredBrain as dealer says a defrag will not provide a way of securely deleting files.
Even commercial wiping software is not 100% guareteed to remove the data as MIT tests have showen it can still be recovered even after heavy software wiping and even degaussing.
The NSA did a test, they wiped at DoD standard multiple times, then sat a magnet on it for a couple of days, degaussed it, took it apart and smashed it with a hammer, they were still able to recover 80% of the original data!
However with wiping up to Guttmann 32 pass standards its very hard to recover any data without the use of a very expencive and time consuming process by which they read the data of the platters with special equiptment, i think they do analysis of the indentations left on the platter or something.
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