if you are REALLY certain you want data to be written over the entire disk I can give commands here that will do it fairly well.
first I would like you to disconnect everything but that one drive and what you have Knoppix runing on, then write back here, yes I am serious
one of the commands is called dd and this command will do a great job of overwriting the information on ANY disk.
my command is actually several commands in sequence that will take a while to complete (we talk days with any large drive).
fact is that some of it may actually still be recovered with some care after 10 repeats, clasified data on things like hospital computers require this to be repeated 25 times and to be treated as still not 100% clean.
some authorities (read armed forces, intelligence community...) require disks with sensitive data to be incinerated at 1300°C, by then the disk is a molten blob of metal waaaay beyond curie point.
Curie point is the point where materials loose their magnetic properties and the domains are in free flux, even a bar of steel is not magnetic at that point (blacksmiths use that fact when they harden carbon or "low alloy" steel, at that point the steel is just few degrees below ideal hardening temperature)