Issue adding drive 4 to ReadyNAS nv+ v2

Thanks StephenB​  obviously I am trying to avoid having to do a factory default with all 4 disks in place (as it would require another copy of the approximately 2.6 TB of data to be taken). And yes, I referred to that as (c) / destructive install of the OS.

So I think I understand the behavior now, any hot adding of a disk is destructive (disk will be formatted and new partitions created - followed by a sync). A cold add would be different, i.e. if you power down your NAS remove a disk to (say) add a partition and insert it again before powering it on it would not trigger a format operation and creation of fresh partitions.

I was considering (a) while from the NAS itself, i.e. without adding/removing any of the disks. Based on what we're seeing though, it indeed appears that the NAS somehow does not create the third partition for a reason. Could this be because we would otherwise exceed the size of the md2 volume?

Thinking about the fresh install I performed, I recall it created the volume md2 and next performed a sync. Maybe it created the volume with two drives, adding the third drive through the sync shortly afterwards? That would imply that the capacity of the md2 volume started at the size of a single disk (2.7 TB). I recall as per 

Maximum disk size for ReadyNAS NV+ V2 | NETGEAR Communities 

 that volumes cannot increase by more than 8 TB from their size at creation, nor can they increase beyond 16 TB regardless. Could it be that disk 4 takes us from a raw volume capacity of 2.7 TB to 10.8 TB (i.e. in excess of the max 8 TB increase)?

I always thought this all applied to net capacity of a volume; in that case we would only be hitting the limit if our starting volume size would have been 2.7 TB. I recall having three out of four drives installed in the NAS when I installed it from scratch using the RAIDar software. But not sure now, I think the original logs in /var/log might already have gone?

I believe it would be helpful if we could see whether we are hitting a maximum or not somehow from a log. I.e. an explanation for why the third partition of a hotly added disk never gets created.