Can I attach an NAS or external hard drive with ethernet port through ethernet to my wireless N router and then access the files on the NAS wirelessly?
I am wondering how to set up to access files on an external hard drive wirelessly.
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While it would be more useful knowing the specifics of your situation, what you are trying to do should be perfectly fine in the general case.
The NAS should work just as well (except of course for the reduced speed of wifi) whether you access it through ethernet or wifi, the physical mediam should be transparent to the IP layer of your network.
Assuming your wireless network is already set up and working, just plug your ethernet NAS into the router and follow it's setup instructions.