ReadyNAS NV+ HD upgrade failed after 4th drive

You dodged one when you were able to back up your data. As one who has lost all his data in the past (joined the lost data club decades ago), you now know why we beat the drum of maintaining a current and complete backup. Never ever trust your data to one device be that device be a single HD or a single NAS.

I know that backing up to external devices is a PITA, especially with today's larger arrays, but you survived what could easily have been a disaster. If you can see your way in the future, add another NAS to your setup and then back up NAS to NAS. I finally got to that point two years ago and enjoy the comfort of a nightly automatic backup of all 2.4TB of data. (I also do a periodic external backup to a WD Passport of all my critical/important data, all but the videos and music, which then goes offsite in case of a disaster). A current and complete backup gives you a lot of leeway when working with problems.