You mainly use an internal hard drive to save movies, pictures, videos, and all your other files. Different devices have different requirements for their internal hard drive. For example, a laptop's internal hard drive can't get too warm, or it will damage your laptop. An NAS drive must be able to withstand 24/7 use, otherwise it won't last very long. Any device that needs an internal hard drive has its own usage requirements. Internal hard drives come in 2.5-inch size for laptops and other small devices and 3.5-inch size for desktops and NAS servers.