Answer Description

Degaussing a disk will destroy it, so that is not an option. Re-partitioning or formatting the disk will mark areas on the disk as empty, but will not destroy the data until that space is needed for new data. Using a tool to overwrite the data, preferably multiple times, is the only way to ensure that data cannot be recovered while still allowing the disk to be reused.

Wikipedia

In computer data storage, overwriting is the process of replacing old information with new information Overwriting generally occurs when unused file system clusters are written upon with new data, though overwriting is also used in security algorithms These algorithms use a precise set of rules to remove any part of original data from the memory by writing new raw data on the memory Data that has been overwritten, even just once, is generally considered to be unrecoverable