So here's what I done, in order:
(one HDD only in my DNS-325)
1) deleted important doc on the NAS (my mistake

) - found out too late about the existence of the Network Recycle Bin

2) to recover the doc, I pulled out the HDD from the NAS and attached it as physical drive to an Ubuntu Virtual Machine; made a full back up just in case (thank God I did it!)
3) successfully recovered deleted doc using Photorec
4) plugged back HDD in the NAS, but now the NAS won't recognize Volume_1, it thinks it's a new HDD and calls it Volume_2
What do I do to avoid formatting and setup anew with Volume_2? All the data is still there, I would hate to have to copy it from the backup drive back to the NAS.