NAS (Network Attached Storage) are hard disks connected to your home or work computer network and unlike USB or Thunderbolt external hard drives, do not have be connected to your Mac or PC directly. NAS hard drives are normally connected to a network switch or router and can be seen by all Macs and PCs connected to the network. The additional benefit is that you can configure them to be accessible as cloud storage. This means you can access all your data from any location which has a fast internet connection. It can work as if you have your own Dropbox provided you have a fast and solid internet connection at home or in your office.