Shredding perfectly good disks is a waste of hardware and probably a warranty violation.
I vaguely remember a warranty disagreement between Dell and the US military... They could hardly return the machines intact after they'd used them. They came to a rather Pyrrhic compromise: Dell got the drives back, but didn't get their platters!
It of course depends on your security standards but a simple dd if=/dev/zero of=/dev/disk will wreck the data badly enough that you'd need to hire cleanroom spacesuit people and their special machine to have any hope of retrieving it. With 7 overwrites, not even that.
The
shred utility I mentioned earlier does that kind of destructive overwrite at the file level, on filesystems that support it, letting you securely delete a file without needing to reformat. Of course that's no guarantee there's nothing unwanted in unlinked sectors somewhere.