a normal HDD will never emulate a real NAS drive as that is a completely different animal!
not sure what you mean by that though lee.
i think of a nas as just a dedicated pc running as a headless server.
usually you need 1 dedicated drive for the os only to run on (some people put the os on a memory stick.) and then you also need other drives to store the data on. the drives themselves can be any old drives, but of course for reliability you'd probably want to use enterprise drives like wd reds.
to be honest though in my opinion unless you need more storage space or redundancy, then i don't see any real advantages in trying to set up a nas over the solution your using now, besides the fact there's no way to have nas storage without actually building/using a dedicated pc of some type (rasberry pi's are popular i believe.) to run the nas software on and connect the storage drives to first.
if you want to build a low powered nas though then here's a couple of links might be of interest to you :-