Darik's Boot and Nuke is a free data-erasing utility that can wipe your entire hard drive and overwrite everything on it with garbage data. Overwritten files are almost impossible to recover, so you can safely use DBAN before you sell your computer to ensure that no one can recover any sensitive information.
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You can install and run DBAN from external media -- CDs, DVDs or flash drives.
Installing DBAN on a USB Flash Drive
Open the Universal USB Installer Web page and click the Download UUI button to save the utility on your hard drive.
Double-click the file and click I Agree to agree to the terms and conditions and then select DBAN 2.2.X from the Step 1 drop-down box.
Click Browse in the Step 2 section and select the DBAN ISO file you downloaded from the official website.
Select the USB flash drive from the Step 3 drop-down box and then check the We Will Format... box to format the drive.
Click the Create button to install DBAN on the USB Flash drive.
Using DBAN to Erase the Hard Drive
To avoid erasing important data from another hard disk, you can remove all hard drives except the one you want to wipe, and then reconnect the other hard drives to the computer later.
Restart the computer, change the boot order from . The DBAN flash drive is a bootable drive.
Press Enter to start DBAN in Interactive Mode -- it prompts you to choose various options before erasing the drive. If you want to erase all hard drives immediately -- not recommended -- type autonuke and press Enter.
Select the hard drive to erase by highlighting it using the J (Up) and K (Down) keys and pressing Space. Make sure the drive you select is the drive you want to erase by looking at its size.
Press F10 to start erasing the hard drive if you are absolutely sure that you want to erase everything, and you understand that the data will be irrecoverable.