What is the MOST effective method for destroying data on HDDs from old desktops?

Degaussing is the most effective method for destroying data on hard disk drives (HDDs) from old desktops because it uses a powerful magnetic field to disrupt the magnetic patterns that store data on the disk platters. When HDDs are degaussed, the magnetic data becomes unintelligible and unrecoverable, effectively rendering the storage media useless for any future data retrieval or access.

Other methods, such as drilling or hammering the drive, physically damage the hard drive, but they may not guarantee complete data destruction or could leave fragments of the magnetic media intact that might still be recoverable with advanced techniques. Low-level formatting, while it can erase data, typically only removes the file system structures, allowing potential recovery of the underlying data using specialized software. Degaussing, therefore, provides a higher level of assurance for complete and irreversible data destruction compared to these alternatives.