ReadyNAS Duo v2 - can't add a second disk

You factory defaulted this unit in Flex-RAID, not X-RAID. X-RAID would expand your disks to 2 disk expansion. As such, you're currently using a RAID 0 and can't expand that. You should backup your data, factory default, and use X-RAID if you want redundancy. If you want another RAID 0 and possible risk of losing all of your data, then you should factory default with both disks in place and use Flex-RAID to choose your RAID option. Your other option right now is to add the new disk as a new volume. 

The messages you are getting about resyncing arrays is because we add the OS and swap partitions to your new disk and resync those for redundancy of those options. We wouldn't do anything with the data volume area in Flex-RAID.