If it's jailbroken you can install XBMC or Plex and watch anything that's accessible on your network.
If the external hard drive is mapped to a computer that's turned on and accessible or if the NAS is configured properly then no problem.
The ATV1 can use its own USB port but the USB ports on the ATV2 and ATV3 are client only.
The USB ports on the ATV2/3 are for diagnostics and re-loading the OS should it become corrupted.
And as long as your NAS is up and running XBMC should be able to access it.
Not sure about Plex but I don't think that XBMC has any special requirements.
I'm running mine in a Windows environment but I'm sure that OSx should work similarly.
As long as your computer can see and access the NAS then the ATV should be able to also.