2016-12-16 - After having learned a bunch more, this has become a much more complex topic. See Data sterilization for bits and pieces of insight.
The secure erasure of data.
I've also heard this called "data sanitization".
NOTICE - This information IS BAD. Other methods must be used to actually securely erase hard drives. I'm investigating them.
NOTICE - Sterilizing SSDs are a totally different topic than sterilizing hard drives. I wouldn't trust an SSD to be securely erasable at all. For an SSD, use fire. Yes, this means that you cannot give, sell or return an SSD if it had sensitive data on it.
TODO - split this into three documents: file, partition, drive
Data recovery, the opposite.