you can use following tools -
if disk size is large , eraser will be right option (
http://eraser.heidi.ie/) . it has DOD compatibility to choose no. of passes and method. Its open source freeware & being 32 bit will make process faster.
For smaller disks you can opt for active kill disk free version.
Technically speaking ATA disk eraser is recommended , HDAT2 can do the same job ,however you must have adequate powerbackup otherwise disk may get ATA password.
Some other alternatives are -
Manufacturer specified tools -like for seagate seatools etc ,
MHDD
HDDSCAN ( You can use it on USB/SCSI/Firewire/ SSD's).
Thanks to greatest contributor to this forum Artem Rubtsov. I salute him for making this available as freeware.I take this opportunity to highlight exceptional technological skills of our russian friends as most top end tools are developed in russia.
Hopt this helps