Darik's Boot and Nuke (DBAN) is a self-contained boot disk that securely wipes the hard disks of most computers. DBAN will automatically and completely delete the contents of any hard disk that it can detect, which makes it an appropriate utility for bulk or emergency data destruction.
There are some wipe methods, they are Quick Erase, RCMP TSSIT OPS-II, DoD Short, DoD 520.22-M, Gutmann Wipe and PRNG Stream. Where DoD is The American Department of Defense. The default is The American Department of Defense 5220.22-M short wipe (DoD Short).
It is a very easy to use utility. A 300GB hard drive will take about 2 or 3 hours to wipe with the DoD Short method. The hard drive after the wipe is like a brand new one.
You can download it at here. Then, burn it in a CD-ROM or create a bootable USB stick with UNetBootin.
That's all! See you.