What is this RN204 doing?

Having just gone through a similar traumatic experience within 2 days of owning my RN204, I can offer a completely technical "it has goofed" comment but looking at the configuration, what that says to me is that the RAID 5 set is degraded, now running on just 2 and 4, being represented by the 'data' volume which is status green.

Meanwhile, the extra space on the mirror (RAID 1) on the two 4TB drives is represented by 'data-0' but is offline.

As an equally RN204 inexperienced user, I feel that the "remove inactive disks" message isn't a good one at all, if that were on one of the software outputs where I work, i'd slap a JIRA in for that immediately.  I would also love to see more detail on what's going on at times because I had 3x1TB and a single 500GB.  I replaced one of the 1TB (non HCL) to one that is on the HCL and resync took about 18 hours.  Then, yesterday I replaced the 500GB with a 2TB drive and the resync is 36 hours.  I can't understand why.  There's no opportunity for it to create a new volume such as yours with the RAID1, all it had to do was shuffle and do parity to spread an even 1TB across.

What I would say is that Netgear 3rd line support remoted into my NAS and fixed my completely inaccessible volume so my advice would be to let it do its thing and see what the end result is, if it doesn't sort itself out, get onto Netgear and see if they can rebuild the data structure.  This is my concern about my unit right now, resilience of its data structures with regard to how the disks and volumes are consumed.

(Sorry, didn't want to hijack the thread, just as equally interested)