ReadyNas Ultra 4 disc failure replacement issue

Contact support.

You might need to clone a disk e.g. using dd_rescue

Support is not keen to assist since it is a HDD issue, not a device issue (under warranty). Would it be easier to backup let's say 4TB or so to another NAS, then take both disc 2 and disc 3 one by one out in order to split the data over the two remaining (healthy) HDDs, before reinserting a new third HDD and after that one works with redundancy a fourth new HDD? How could this be done? How do I make sure all remaining data is only stored on disc 1 and 4? Please advise.

MarcoScholten wrote:
How do I make sure all remaining data is only stored on disc 1 and 4? Please advise.

You can't. You can back up 4 TB to another NAS, and 3 TB to the WDC Red that isn't part of the array (disk 3).

But if you remove disk 1, disk 2, or disk 4 you will lose all your data. Of course you need another drive anyway, so perhaps buy a 2nd WDC Red ASAP, and start backing up what you can while you are waiting for it to arrive.

How full is the volume?

Getting a bit anxious now, since the last message stated: "Reallocated sector count has increased in the last day. Disk 2: Previous count: 45864 Current count: 46120 ATA error count has increased in the last day. Disk 2: Previous count: 256 Current count: 290 Growing SMART errors indicate a disk that may fail soon."

Every time I reboot the system with the new 3TB WDC Red in disc 3 it comes up with "resync c: 0.0%" on the Ultra4 with the percentage increasing, but on finish it fails, marks disc 3 in frontview as "spare" and the Ultra4 goes on life support :(

I had 7.5TB in the array on disc 1, 2 and 4, but am now backing up 4.5TB onto external drives and wish to backup the remaining 3TB on the new 3TB WDC Red that isn't part of the array just yet (disc 3). Fingers crossed I'm able to save as much data as possible before disc 2 fails.

Question is: How do I backup 3TB to the WDC Red that isn't part of the array (disk 3)?

You will need to remove it, put it into a drive caddy and set it up as you have done with the USB drives you are using, then do a backup.
As StephenB has suggested, you might want to power down the NAS and clone disks 1,2 and 4. Of course you would need additional disks for this but if your data is important it may be worth it.