We all come across a time where we need to get rid of a computer that has a hard drive or just a hard drive itself with your data on it. So what do you do with the data in it? If you delete from the OS, there's still possible ways to recover the data from the next person that the hard disk goes to especially if you just do a quick format of the disk. Doing a quick format essentially removes the pointers to the data. You might not be able to see it from the OS, but the data is still on the disk. And with the proper recovery software, the data can be recovered. To be safe, its best to write the disk with random data. When doing so, it's nearly impossible to recover data that was written over. Killdisk, which I've used before is simple and effective. The free version however, is limited to how many disk/partitions you can wipe at once. If you've got a Linux machine, you can use the 'dd' command and fill the disk with data from /dev/urandom. Wiping a disk with random data can take a long time but its the safest way to protect your data from being recovered by someone else.
https://www.killdisk.com/eraser.html
dd if=/dev/urandom of=/dev/sdX (X is the letter of the drive)
