Discrepancy bvetween actual size vice advertised size

If you look here, 

http://rdconfigurator.netgear.com/raid/index.html

Add in your 4 2TB HDD's, you can see you end up with 5.43 Terabytes.   For one thing, a formated HDD that says 2TB is not.  It works out to 1.82 TB.  This all has to be with HDD manufactures saying 1000 is a byte, while Ram, 1024 is a byte.  So when you get into Terabytes, that adds up.    With a 8TB HDD, the size is really only 7.26TB.  Your computer see's everything as 1024 as a byte. I don't recall why things are this way, but people have complained forever.  They put a new HDD in their computer and then wonder why it's smaller then it says on the Box it came in.  Same holds true with the NAS.  

So you end up with 5.43TB of storage and 1.82 for protection in XRaid 5.  Another 214 with 8TB of capacity means it's using larger HDD's. Which using the Calculator, would mean it would have 4 3TB HDD in it.  That gives it 8.16TB of capacity with 2.73TB for protection. 

So you have 4 slots, thing of it as 3 for storage, 1 for protection.  I have 6 slots, so 5 for storage and 1 for protection.  So I have 6 3TB drives in mine.   So that works out to 13.6TB with 2.73TB used for protection.

The most you could ever have with 4 2TB HDD is 7.26TB.  If you did raid 0, which is no protection at all.  If 1 HDD fails, you lose everything.  Though using either raid 0 or raid 5, you should still always have a backup!!!  Having RAID is not a backup.  You could have 2 HDD fail on you, or one fail, you pop a new drive in and as it's rebuilding another HDD takes a dump on you and you lose all your data.  The NAS could break down.  It could be stolen.  Or go up in a fire.  

Advertized size means it does have 8TB as in 4 2TB HDD in it.  If you used 1000 bytes and not 1024 which your computer understands, they would be 2TB HDD.  I'm really surprised in this day and age that people still havn't learned this.  

https://www.howtogeek.com/123268/windows-hard-drive-wrong-capacity/