A network attached storage (NAS) device is a server connected to a network with the sole purpose of providing storage. NAS devices often use a RAID configuration. An external hard disk is a separate freestanding hard disk that connects with a cable to a USB or FireWire port on the system unit or communicates wirelessly. External hard disks have storage capacities up to 4 TB and more. A removable hard disk can be inserted or removed from a built-in or external drive. Removable hard disks have storage capacities up to 1 TB. A disk controller consists of a special-purpose chip and electronic circuits that control the transfer of data, instructions, and information from a disk to and from the system bus and other components in a computer. A hard disk controller may be part of the hard disk on the motherboard, or it may be a separate adapter card inside the system unit.
