I have migrated family members to a new and bigger NAS and needed to purge the old drives for disposal or selling. The Arch Linux Wiki has a good page about securely wiping disks as merely deleting files and/or the partition is not enough for magnetic media.

Shred Link to heading

My go-to tool is shred as it is part of GNU coreutils and thus comes out-of-the-box in most distros. Also, the coreutils are available on brew for both MacOS and Linux. The parameters I use are probably super-paranoid, but as I am not a HDD forensics expert, and it can just run without supervision I am keeping these:

shred -vz -n10

This will do a total of 11 passes:

  • 10 passes writing random data
  • a final pass writing zeros

man page excerpt Link to heading 

-v, --verbose
      show progress

-z, --zero
      add a final overwrite with zeros to hide shredding

-n, --iterations <NUMBER> [default: 3]
      overwrite N times instead of the default (3)