How do you erase a hard drive?
Technically the word erase in this context means to “remove” recorded matter from a magnetic medium, or to “delete” from computer storage, per Merriam-Webster. As a matter of industry practice the word ‘erasure’ has come to mean overwriting in the media disposition industry.
Overwriting is a method where a random pattern of usually ‘1’ and ‘0’ is written to every part of the drive, frequently three times, to obliterate the underlying data. The advantage of this method is the drive can be re-used, and for most practical purposes the original data is not recoverable. The disadvantages of this method are: it takes a skilled operator to ensure that the overwriting is done correctly for a given manufacturer and version of the drive; it takes a considerable amount of time-hours, and therefore electricity to do; and it is not regarded as failsafe, or permissible for the highest data security standards such as the NSA, CIA, or DoD.
In a broader context of the word erasure, there are other ways to remove the data from a magnetic device, or to make it inaccessible. Many entities encrypt their data, and then you only have to destroy the key to the algorithm, and the data cannot (theoretically) be recovered, making it inaccessible.
Another permutation is Secure Erase, which is an overwriting protocol that is built into some drives at the factory and is supposed to work like an external overwriting program mentioned above. In theory, using the factory installed Secure Erase function alleviates the need for a specially trained technician to match the program to the drive/version; in practice about a quarter of Secure Erase programs have proven to be faulty and drives were released with data intact.
The optimal solution is to degauss the magnetic storage device as degaussing eliminates the data-there is nothing to recover as the magnetic field that was the data, simply no longer exists. This renders the drive un-useable as all the track and sector information is gone as well.