networking - how can NAS work if it is not connected to Internet?

I was reading the reviews for a NAS backup solution: Western Digital 4TB My Cloud Home Personal Cloud Storage - WDBVXC0040HWT-NESN. I am referring to the this review on the product that criticized the solution on the basis that it wouldn't work without being connected to the Internet. The idea was that it should be possible to disable connectivity to prevent the backup device from being hacked remotely (and, presumably, added to a botnet).

I'm not very familiar with network hardware so this didn't make much sense to me. For the backup to work from a laptop to the NAS device, some sort of connectivity would obviously be needed. The device is presumably plugged into the router using an Ethernet cable, and the data flow presumably goes either laptop->router->NAS or laptop->router->some_server->router->NAS. Is the idea that the former should be possible but that the device is in fact doing the latter?

I realize that it might be hard to judge this device from afar without using it yourself, but your best guess as to what's going wrong with this device would be helpful.

Answer

The Ethernet ports in your router are just a switch, basically. The home network does not need internet to work internally. The router will still assign IP addresses, if the option is enabled and the network will still work. You just cannot connect to the internet or to your router from the internet. This is why a NAS like this will still work, but only on your internal network, which means only via WiFi provided by said router and from devices connected via Ethernet (LAN) cable. No access from the internet will be possible. This includes phones not connected to the internal WiFi.

NAS means Network Attached Storage, which means, that the device is, in it's essence, a small computer with a connected hard drive. So all the storage remains locally and is, normally, not uploaded to the Internet, unless configured otherwise. The NAS is sharing access to the device via network folders and, most commonly, uses the SMB Protocol. Most NAS also provide other means of access, such as File Transfer Protocol (FTP) and various others.

If this NAS is not allowing usage without Internet Access, then it is a serious flaw that should not exist and flat out a bad product. It is essentially doing what is commonly called "phoning home".