Other than the type of box it’s in, I really don’t understand the difference.
An external hard disk can only be connected to one PC at a time. An NAS is connected to the network, and can therefore be accessed by hundreds of devices simultaneously. An NAS also generally has an OS built into it and additionality functionality not available in an external drive, such as streaming movie player, printer sharing, or remote access over the internet.
In fact, an external hard disk is no different to an internal one. the box it's in simply provides power, and changes the SATA /IDE interface to a USB. No additional functionality or brains in there.
Thank you James.
I assume then, that the NAS OS is there mainly to take some stress off of the accessing PCs, is this correct?
Hmmm... That's a good way to think of it if the box has some kind of built-in functions, yes. But, not all NAS devices have functions beyond just a networked hard drive though, so it may be just a very light OS that enables it to operate on the network. So it *can* take some of the strain from the PC, but that's not why you would use one because the benefits would be minimal. The main point is that is enables access over the network, and hence by any other machine on the network. Perhaps it's better just to think NAS = file server.
Beyond that, we'd have to start talking about specific devices. For instance, there are NAS devices that are basically just an external drive, but they happen to work over a network cable. Some routers also include basic NAS functionality already in them - you just need to connect a regular USB drive and the router becomes a NAS! And then you could build your own NAS device using an old PC and a linux based operating system like FreeNas.
Were you trying to decide which to get? An NAS or regular external disk?
You can do this with an external hard drive connected to a router with a USB port - automatically networks the hard drive and makes it accessible to everyone (no password login though that NAS provides better security). Example: External drive + Apple Time Capsule. You can use Time Capsule for Time Machine and your NAS drive connected via ethernet for iTunes, DLNA and external login security. Look at what you need before you buy NAS to make sure you really need it.
It's more like a very low profile computer with a stripped down OS (mostly some Linux). The OS is there manage the storage (Partitions, File System), to provide network connectivity (TCP/IP, settings etc.) and services (SMB, AFP, ...). Without those it wouldn't be possible to access the device.
It's not about taking stress off the accessing device. No matter if the storage is NAS, SAN or DAS (direct attached storage via USB/FW/eSATA) using the storage will result in activity (as in used resources) on the computer.
In general you can say that an external Hard Drive is really just an Hard Drive with a converter to one of the external interfaces (USB, Firewire, eSATA) while a NAS is a special form factor of a server with a certain set of functionality.
The only storage that sits in between would be NDAS (Network Direct Attached Storage). You can see it as a server-less network attached Hard Drive. It also doesn't use TCP/IP but the LPX protocol for access.