Delete A Hard Drive Securely

Leave No Trace: How to Completely Erase Your Hard Drives, SSDs and Thumb Drives - Data wiping - Gizmodo
Conclusion

We've highlighted a variety of free ways to protect data on castoff drives from being retrieved. As you can see, your best bet is to overwrite data directly, but you also might want to consider using a program such as SDelete to scramble filenames first and then use a disk wiper such as Eraser or WipeDisk to finish the job.

Comments: The article is long so here is what I do.
1. DBAN if your OS is trashed and you can not run windows or you wish to delete the OS drive. This simply boots and nukes the drive, thus the name. When? If discarding the drive or you want to still use the drive but be rid of any previous information.
2. DiskWipe if you can run from windows and wish to delete the none OS drive.