To completely wipe a hard drive for free, use a free third-party destruction program such as Darik's Boot and Nuke, or DBAN. Alternative software for the same purpose is Active@ KillDisk. Before attempting the wipe, ensure to back up any important data currently on the hard drive.
To wipe the hard drive using DBAN, start by downloading the program's ISO file from DBAN's official website. After successfully downloading it, burn it to a disk or transfer it to a bootable USB drive. Insert the disk or the USB drive into the computer with the hard drive, and reboot the computer. Ensure to boot into the disk or USB drive.
Press F3 when DBAN's main menu appears to open the Quick Commands screen. The Quick Commands screen lists the commands that can work in the boot prompt. Each command uses a different wiping method. Proceed with caution, as each command wipes disks on the computer without confirmation.
If you prefer the interactive mode to working with quick commands, press Enter instead. Choose the wiping method, select the hard drive you wish to wipe, and then press F10. Wait for the erasing process to run to completion. After the wipe is complete, a message confirming successful erasure appears.