Andrew misses FDisk in MSDOS. He liked typing from the command line. Leo says that FDisk still exists in Windows, and when he deletes a partition with it, the data isn't lost, it just loses the structure of partition information. If he wants to erase all the data, Leo recommends Darik's Boot and Nuke (DBAN). It erases the data, overwrites the hard drive, erases it and overwrites it again. Seven times. So there's no way the data can be recovered.
But DBAN won't work on a Solid State Drive. It uses a feature called Wear Leveling, which protects the sectors and some data leakage can occur. That's why Leo recommends turning on whole drive encryption. Then, when he deletes the key, the drive is unreadable.