RN626X: Data array missing from mdstat after OS reinstall (password recovery)

What you are seeing is the inability of the OS to combine two RAID segments into one.  Each of the "volumes" you see is a fragment of /data.  Since there cannot be two volumes named /data, it names one after the MDADM RAID on which it resides, data-0.  This is probably not an artifact of the re-install.  It is most likely an additional effect of the corruption that made your password not work.  DO NOT delete either, or you data will become unrecoverable.

The best choice is to simply factory default and restore data from backup.  If you have no backup, manual re-assembly is the only other choice.  If you have a place for them, you could image the drives as a precaution.  There is recovery software out there that can do the hard work for you, but StephenB​ can help guide you if you want to just do it with native Linux tools.

Even if you successfully recover the volume, it's content is going to be suspect and I would not trust it to stay intact.  It may even automatically change to read-only if the corruption warrants.  So you will need to back up the contents, factory default, and restore the data as a final step -- ideally before you have written anything new to it.  While that's going to be expensive if you don't already have the means to do a backup, especially with the current price of hard drives, it is a start to having the backup you should already have.  You may want to consider using a newer NAS for that so you can move away from the unsupported ReadyNAS as your primary storage and relegate it to backup.