Without a computer or another enclosure surrounding them, these hard drives are called “internal hard drives.” They’re small metal boxes akin to old-fashioned record players, with one or more spinning disks (“platters”) that get accessed by a “ read/write head” (shown above). The vast majority of Macs in homes have mechanical hard drives (rather than chip-based SSDs) inside. Photo credit Eric Gaba, Wikimedia Commons user Sting A Quick Primer On Internal Hard Drives