Get the why and the how on erasing hard drives in this article.
Gizmodo has a great article on why you need to completely erase hard drives, USB drives, and SSDs and serves up several ways to get the job done. SSD is (in this case) an abbreviation for Solid State Drive, a technology that has been under development and has been making its way into the consumer arena recently. Along with how to erase drives the article has software suggestions, with a section on utilities for certain brands of hard drives; Hitachi, Samsung, Seagate (including Maxtor) and Western Digital.
Other highlights in the article are securely wiping a hard disk, wiping flash memory cards and USB drives, and wiping SSDs. There's also a section that evaluates the effectiveness of disk wiping programs.
If your hard disk dies and can't be erased, I can recommend removing it and using a sledgehammer on it. Really.
Leave No Trace: How to Completely Erase Your Hard Drives, SSDs and Thumb Drives
Signing off for now,