My QNAP TS-412 had a major issue, 2 of the RAID5 disks decided to fail at the same time. Luckily QNAP was still up and running and allow READ-ONLY access to my data. However, all the iSCSI LUNs are not accessible anymore from my iSCSI clients.

Found the attached document which basically helped me recover my iSCSI LUNs. I copied by iSCSI LUN files to the backup locations and rebuild by NAS volumes. I then follow the procedure #3 in the document to get my iSCSI LUNs back.