New Disk added to slot 3 and its now not working correctly.

I used to have one of those NAS units a while ago.  I had 4 3TB WD Red drives in it. I started with 2 myself which puts it into a Raid 1 format.  Which means everything put 1 one hard drive is also copied over the the second hard drive.  When you put in a 3rd hard drive, I put in a 3rd 3TB HDD that matched the first 2, it changes over to a RAID 5 format.  So that if any 1 HDD fails, the data is still on the other 2 drives.  When you pop in a new HDD, it takes HOURS or longer depending on the drive size for the NAS to do it's thing and make it part of the overall system.  When I was getting full again, I popped in a 4th drive and it took hours again to incorporate that new HDD.   I didn't have any problems.  

Since then I upgraded to a ReadyNAS 516 which can hold 6 HDD.  So those Orignal 4 HDD from my old NAS are in this newer NAS and were the first 4 I installed.  I have a couple HDD dating back to 2013 in it and are just fine and error free.  So just over 4 years old now.  I then added a couple 3TB Seagate NAS drives for the other 3 spots.  I'm also full. I do have have a brand new 3TB Seagate drive around here someplace for a spare.  It takes a long time for the HDD to become part of the system. Even then, you have to realize how much space you really have.  Starting with the 2 1 gig HDD in Raid 1.  You end up with 922 Gig's of space on your NAS, when you throw on a 3TB HDD, it only goes to 1.8 TB. When you add that 4th HDD, space jumps up to 4.51TB.   Some people think something might be wrong and pull the HDD and screw things up.    It's those 1TB drives that kind of screw you over.  I have the same problem if I want to jump up to say a 6TB HDD to double my space.  The 3TB drives I have kind of bring me down.  Remember if say 1 3TB fails, there's only the other 1 3TB drive and the 2 1TB drives in your system.  The NAS has to be able to restore the Data no matter if a smaller or a larger HDD fails.

A NAS is also not a Backup solution.  For myself, I have a couple Seagate 8TB Archive HDD.    So I backup all the Data on my NAS to those 2 drive. These drives are really only good for backup because of the way they store data on them.  But they're resonably priced. With my 6 3TB drives, I have 13.62TB of storage space.  So 2 of these are enough to backup my NAS with room to spare.  You can get one of these Seagate 8TB Expansion drives which plugs in at USB3 for cheaper then the same bare 8TB Archive drive!!!  Which is what I ended up doing and assumed was inside it.  I was right when I opened it up. I didn't want the external case and the interface.  Not for my needs.  I have HDD docks where I can plug in a bare drive.  When done, I pop them into a plastic HDD case and pop those drive into my Fire Resistant Safe.  So I now only have a real backup, but it's pretty safe from Theft if someone breaks into my house or if I have a fire.  I shouldn't lose all my Data.  So I got this Seatgate 8TB drive from Amazon.  I'm looking right now andit's even cheaper now at $179.99.  It's the Seagate Expansion 8TB Desktop External Hard Drive USB 3.0 (STEB8000100).  They have smaller sizes. 5TB is $119.99.  That single drive should backup what you have now.  If you get the 8TB one, would backup if you swapped out those 1TB drives later for 3TB ones.

By the way, $219.98 for the bare 8TB hard drive!!!  So you save $40 and get a external case with a USB3 controller and power supply.  It's the exact SAME 8TB Archive HDD.      So a NAS is not a Backup.  If you're backing up your computer to the NAS, then it is a backup to the Data on your computer.  But if you have other stuff on that NAS and it's no where else.  That's not a backup solution.  How about if you have some type of issue, like you do NOW.  How about for whatever reason the batch of HD you got were not good and 2HDD failed at once, or 1 failed and during the rebuilding of your NAS for that new HDD, another HDD fails. You've lost all you data!!!  How about of the NAS just fails hardware wise for whatever reason.  how about if you get Robbed?  How about if you have a fire?  There is so many things that can go long and the more Data you have on your NAS, the more Data you could lose.

Hopefully Netgear will be able to help you by looking in your Logs of what the issue is and can fix you up.  They are generally pretty good in doing that for people.  They've been a great help for me here.  The NV+ V2 which I had was a slow, poky NAS.  But it was a good starter NAS back when I got it.