The internal hard drive is a crucial part of a NAS, at least, if the NAS doesn't include any hard drives by default. In most cases, you'll purchase a NAS that doesn't include and hard drives. When purchasing hard drives for your NAS, keep in mind that the drives have been approved for use by the manufacturer, in the model that you're looking to buy. These disks can withstand the slight vibration that a NAS emits, making them last longer.