'Shredding' a hard disk

[Philipp Lehman <lehman@gmx.net>]

I want to sell an old machine that I don't use any more but there's 
still sensitive stuff like personal emails, gpg keys, and passwords on 
the hdd. What's the recommended way to securely 'shred' this data so 
that it cannot be undeleted by the next owner?

I assume I need to do something like overwriting the whole drive 
serveral times with random data before finally erasing and reformating 
the whole drive. Are there any apps out there that do this or do I have 
to roll my own solution using something like "cat /dev/urandom | dd"? 
Any suggestions?

Any pointers and suggestions appreciated.

-- 
Philipp Lehman  <lehman@gmx.net>


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