HITACHI 3TB XTERNAL to Internal secondary HDD

External hard drives are just ordinary (internal) hard drives fitted inside a fancy-looking enclosure and connected to an internal sata-to-usb converter circuit board.

So indeed they can be fitted internally, but the hard part is often getting the drive out of the enclosure since no screws are used. The case is either clipped tightly together or bonded. It's therfore invariably necessary to damage the enclosure in order to extract the drive.