I would very much welcome your advice on best next steps. I have considered the following but recognise only too well that I may be way off track:
(1) Add a DX517 extension unit and incrementally add further disks as required. I had hoped that I could use two of the additional five bays to serve as a backup to the remaining bays (four on the NAS and three from the DX517). Can I do that if I set the extension unit up as a separate volume?
(2) Replace the 4 x 4TB disks with, say, 4 x 8TB disks. This seems likely to be prohibitively expensive.
(3) Remove RAID completely from the NAS, thereby giving me near to 16TB disk space. Coupled with the removal of RAID, I would continue with the attached external HDDs for backup, rotated regularly.
If you have RAID6 (SHR2) on your DS918+ 4-bay NAS, the first thing to do would be downgrading RAID to RAID5 (SHR1). This will give you the extra capacity of one drive.
The next step would be getting expansion unit DX517 (). Aditional drives from expansion unit can be added to your existing RAID keeping your system still stable. One or two drives on the expansion unit can be used as a backup, but it is safer to have backup which you can take with you or store in different location.
Replacing drives to the bigger ones is now little too late unless you want to have separate volumes (you might be able to combine raids into one volume). Many people start with just 2 or 3 drives of maximum capacity (12TB) and add extra drives later.
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