Dead volume after disc replacement

Looks like a dual disk failure indeed. When you replace a disk, the NAS has to rebuild it, that is a stressful operation for the other disks. If the disks are healthy, then there shouldn't be any problem. If you have a dodgy disk, then it becomes a risky situation.

A good way to avoid running into this is (only applicable if the disk is dying but still in sync with the RAID array - not dead):

1. Shutdown the NAS.

2. Pull the first disk you want to replace.

3. Clone the disk.

4. Insert the cloned disk in place of the dead one.

5. Boot the NAS.

6. Check that everything is fine.

In your earlier situation, when you know two disks are dying, the best approach I can think of would be:

1. Shutdown the NAS.

2. Pull the first disk you want to replace.

3. Clone the disk.

4. Insert the cloned disk in place of the dead one.

5. Pull the second disk you want to replace.

6. Clone the disk.

7. Re-insert the second disk you wanted to replace back in the chassis, not the clone, the original one.

8. Boot the NAS.

9. Check that everything is fine. If so, continue.

10. Shutdown the NAS.

11. Replace the second disk with its clone.

12. Boot the NAS.

13. Check that everything is fine.

That's what I would do, but it's open for debate ;)

Anyway, you're past this situation.

What you should do is try clone the disk 4 as it's the dead disk that it's the most ouf of sync.

So:

1. Shutdown the NAS.

2. Clone disk 4.

3. Insert the clone of disk 4.

4. Boot the NAS.

5. Hope that the RAID will start.

If the RAID doesn't start, there may be a need to force it, but I can't advise you to do that.

If it does, it will rebuild disk 1.

You can contact a data recovery company, or even NETGEAR offers this type of service, but it isn't free. They can do more than me (a user on a forum), they can assess the situation, take precautions and confirm the necessary steps according to the exact context.