Use an external dock or enclosure to mount the HDD as external.
Download and install killdisk, a free utility that allows you to zero the HDD, rendering it to factory settings.
In Killdisk, select the root of that particulat HDD (make sure you pick the correct one, otherwise you could wipe out some other drive) and run the erase function, according to the wizard.
Voila, you have a "brand new" old HDD!