A NAS isn't actually distinct from a plain drive in that particular way. A plain SATA hard drive is a cheap computer with some attached hardware, just like a NAS. Literally, there's probably at least one ARM CPU core in there controlling everything and communicating over the link to your main computer.

The difference is the protocol that goes over the link. In a NAS, it's SMB/NFS access to files over ethernet/wifi, and for a SATA drive it is the SATA command set accessing blocks. The difference is small.