Remove inactive volumes to use the disk. - RN314

Doesn't look like you put your smart_history.log in the attachment but with those error counts it would look like disk 3 has been failing for some time.

If you don't have a good backup this is a data recovery situation though if you destroyed the volume NETGEAR may be unwilling to try a data recovery attempt. They do sell data recovery contracts but they do software recovery only. They don't repair broken disks. Disk cloning costs are additional if you don't do that yourself if that's needed.

You could alternatively try 3rd party software such as ReclaimMe

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di0deman wrote:

 

You now have a "data-0" volume and a "data" volume.

The "data" volume appears to contain no data, so I accidentally erased the "data" volume.

 

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That is unfortunate.  The system was seeing two broken partial BTRFS arrays instead of one whole one, and no two arrays can have the same name.  So, it used data-0, which is the name of the MDADM RAID, for the second.  But you erased one of them, which means you at least tried to erase even more of the data than what was already missing due to the bad drive.  If you succeeded in the erasure, then you further reduced any chance of recovery.

Step one is to stop trying to fix things.  Anything you do is likely to reduce your chances of data recovery.  I don't know if the products that claim to be able to recover data from a ReadyNAS can deal with what you've done (assuming anything really did happen), so starting with Netgear support is probably your best next step.