Opening your computer case just to disable an internal hard drive can be a complicated affair; it's much easier to simply disable the drive from being detected at the hardware level. All you need to do is access the Basic Input/Output System (BIOS) and tell it that no hardware is connected to that particular set of wires. The BIOS is responsible for dictating how your motherboard operates with the hardware connected to it.
Power on your computer and press the "F2" or "Delete" key when you first see text appear on the screen. This will open the BIOS menu. If neither of these keys work, look closely at the text when the computer first starts. It will tell you the keyboard key to press.
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Navigate using the arrow keys on your keyboard. Each BIOS is different, so reference the legend located on the screen to assist you in navigation and making changes.
Find a menu labeled "Hardware" or simply browse each one until you see a listing of drives. They are typically labeled by their model number (see Resources).
Highlight the drive using the arrow keys to navigate and press "Enter" to get a listing of options for it. Highlight "Disabled" or "None" using the arrow keys and press "Enter."
Navigate to the "Exit" menu and find an option typically labeled "Save settings and exit." Highlight it and press "Enter." You will be asked to confirm your choice; do so. Check the legend of your BIOS as well, as this option may have a shortcut key; a common one is "F10."