Methods for erasing data in disks

The article at: http://www.computerworld.com/article/2477228/dataprivacy/is-overwritten-data-really-unrecoverable-Thtml inquires this. A scholar paper “Secure Deletion of Data from Magnetic and Solid-State Memory” by Peter Gutmann cited in this article explores this idea further. Read Gutmann’s paper and describe what method(s) exist to recover erased data in disk in your own words. The paper can be found at the URL: https://www.usenix.org/legacy/publications/library/proceedings/sec96/full_papers/gutmann/index.h tml