RR3312 Copying Errors

Well, then I can't point to anything that I think could have caused it, though i suppose there could still be an issue with the NAS hardware.  I can say that it's atypical and I don't think you should be concerned you'll be encountering it periodically.  I've used a variety of ReadyNAS over the years, including my current use of legacy rack-mount systems converted to OS6, and I have only seen this once.  And that was mostly my fault -- the eSATA cable between an RN516 and EDA500 was loose, causing a lot of data issues.  (I say mostly my fault because the choice of connector for eSATA is just a really bad engineering decision -- not Netgear's, mind you, but by whoever designed the interface standard. ) 

Occurrences of similar issues reported in this forum typically have an identifiable cause (usually a failing drive with at least ATA errors or lack of proper UPS protection).

Since I knew what caused it and that it was corrected, I thought I could save the volume.  That's where I got the experience to say don't even try.  No matter what I did, the volume was read-only again within minutes.  Since my corrupt volume was on the EDA, I could just destroy and re-create it and not touch the primary volume.  I feel your pain in the data restoration -- the EDA500 is extremely slow due to the eSATA interface, and I had 5x6TB in it in RAID5.  But it was mostly archival data, so that probably made it a little less painful than yours will be if you are without current working data for any period of time.

The volume rebuild process will also be a good gauge of whether you do have a hardware issue, so check the logs after that completes.