According to man shred, shred will repeatedly overwrite files, but only files which exist at the time shred is invoked. It is designed to securely erase data files, but might miss file fragments from file updates. dd will overwrite even your operating and file systems, after which it will crash.
If you are going to use dd to repeatedly overwrite your file system and data to make it totally unrecoverable, you are going to first, have to be entering your commands in a terminal from a Live install, second, have to make sure exactly where what you want to erase resides on the physical drive, and third, set the parameters of dd properly to erase the data successively. Remember that dd will even overwrite any partitioning as well. Your chances of rendering your hard drive completely unusable is possible.
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