new to NAS looking at 214 and need to know what drives are best to use

I've had really good luck with the WD Red Drives.    I'm using 4 of the 3TB versions of them currantly, the oldest over 4 years old now and still slowing zero errors.  The slower drives are more then fast enough, especally for the 214.  Your limitation is your network speed and not the drives themsevles.  The slower drives will not only run cooler, but will be quiter.  In general there's only the single fan in the back cooling everything.  I think Less heat = longer laster HDD.  You do wany to use a NAS HDD,  not just throw any old thing in.  

I would get the LARGEST HDD you can afford.  I would go 2 4TB HDD over 4 2TB HDD.  Just for the simple fact that you can never have enough space.  You think 8TB is more then you ever need and then you about MAX is out and think, darn should have got a larger HDD.  Why be stuck at 8 when you can go to 16TB!!!

I upgraded my NAS from a 4 bay Home unit to a 6 bay ReadyNAS 516.  I currantly have the 4 WD 3TB Red drives in it that I moved from my old NAS having to reformat them for the new NAS, plus a new 3TB Seagate NAS drive I'm giving a try with plus a spare I can pop in at any time.  Those 5 3TB drives give me a total space of 10.9TB in a XRAID5 format.  I'm down to 1.3TB now,  but I have space for that 6th drive.  After that I can expand to larger HDD.  Start popping in 6 or 8TB sized HDD and grow that way.  But to make use of that extra space I would need to add 2 larger drives.   Not at once of course, but pull one, let the system rebuild onto the new larger HDD and when done, rebuild onto the 2nd larger HDD, at which point I belieave it would expand to add the extra space of those drives.  I'm not 100% sure as I've never done that.  I think that's what happens with XRAID.  I beleave you can expand UP, but not down.