Issue adding drive 4 to ReadyNAS nv+ v2

t1000-forever wrote:

The disk that has been hot added last loses its third partition somehow.

When you hot-add a disk, the NAS first formats it, and then creates the partitions, and finally resyncs.  (This happens no matter how you add the new disk).

So what is happening here is that the NAS is choosing not to recreate the third partition after the format.  The partition is not "getting lost".   Usually that is a result of the NAS deciding that expanding the array is not possible.

I think your best option is to do a factory default with all disks in place, and see if you get different behavior.  That might also resolve your issue with the OS partition RAID group.  You would need to restore from backup afterwards.  Not sure if that is what you meant by a "destructive install of the OS" or not.   

Your option (b) would only have an effect if the OS was somehow corrupted - there is no indication that has happened. It shouldn't make any difference, but it is easy to try.

Option (a) is for a completely different NAS model.  That said, manually creating the third partition and rebooting the NAS might work.  If you do this on a PC, then make sure you power down the NAS before removing the drive, and leave it powered down until the disk is re-inserted.