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Before tampering with your computer, it is important to make sure the computer is powered down and unplugged. Each computer is different.  Consult your user's manual or manufacturer's web page to learn how to open your computer to access the hard drive.  For desktop PCs, you can generally remove the side panel of the computer tower. For laptop computers, you can generally remove the bottom of the computer below the keyboard. In some cases, a hard drive may have a ribbon cable, and a power cable attached to the motherboard. In other cases, the hard drive may be directly attached to a port on the motherboard. If it has any cables attached, detach them.  If the hard drive is connected to the motherboard, pry it loose from the port it's connected to. If you want to salvage any of the other computer parts, be sure to touch something metal or wear static wrist bands to prevent static discharge that can damage the inside of the computer The hard drive is generally housed in it's own compartment.  It may be held in place with screws.  If so, remove the screws and then slide the hard drive out of it's compartment. Unscrew all the screws holding the top on. You will need the T-9 sized wrench for most hard drives. Sometimes there's an air seal. You will need to remove this. Once you get the top off you will see two or three stacked, silver disks (called platters). Put scratches on the surface of the platters with the Torx wrench. Now start smashing it with the hammer. Do this outside on a hard surface (such as concrete).  Be sure to wear safety glasses to protect against flying debris. Glass platters (found on newer drives) will shatter. If you have a large hammer (a 10 lb sledge for instance): you can forgo opening the drive - a few good hits with a big hammer can split open the metal casing and mangle the platters, even on old full-height 5.25" drives with metal (rather than glass) platters. Solid-state drives (SSDs) do not have platters on the inside.  For these drives, you will need to smash the drive as much as possible.
Power down and unplug the computer. Open the computer. Disconnect the cables connected to the hard drive. Remove the hard drive. Take apart the hard drive. Destroy the platters.