[NV+ v2] 4 disks, 2 separate RAID-1 volumes?

Rebuilding a failed RAID array is what backups are for. If your clients data is important and uptime is of importance then a second ReadyNAS is something to consider. The first can be set to automatically back up to the second every night. Once the full backup is don (via NFS) then the backup routine is changed to use rsync and it synchronizes the two data volumes. This is different than the standard backup in that there is not true incremental backup to contend with. With a normal initial/incremental back up scenario you would have a full backup and then additional incremental back ups of files that had been changed. The primary NAS is mapped to the drives letters, but the second is not. If you have a problem with the first unit, you simply rename the first NAS to something different and then rename the second NAS to the primary name and all the maps are in place (since the maps are on the client PCs and are pointed to an NAS name). Then you resolve the problems on the first NAS be it hardware or data. If you need to rebuild the RAID, you simply do so from the (now) primary unit.

FWIW - I use very inventive names such as NAS1 and NAS2 in my case. With firmware updates I never update both at the same time, I leave a minimum of a week between the updates.