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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What can I connect to a NAS?
Wireless Speaker Sonos
The great thing about Sonos is that you don't need to pay for music subscription service. With the cheapest NAS at home like DS115j or any other, you can have your backups done for all of your devices and still use your network drive as a Sonos music library. Sonos will connect to your NAS via your network and let access your provided music library. If you have little more of IT knowledge, you can even set up your Alexa or Sonos with Alexa built in to play music on your voice commands ("Alexa! Ask DS Audio to Play Music!").
Nowadays it is so cheap to upgrade your old TV and make it smart. All you need is Google Chrome or EZcast alternative plugged into your HDMI port at the back of your TV. Now you have great functionality added to your TV to send a video file directly from your NAS to a TV. No need for wires and chargers and other mess. You can use your phone as a remote to go to your movies or family videos and cast that content on your TV during family or friend gatherings.
Amazon Fire TV
You may not have a smartphone, or you just simply prefer to control your content with a remote in that case Amazon Fire TV will be a great addon to get to your TV. It also plugs into your HDMI port and will play content from your NAS to a TV. No wires no mess.
nVidia Shield TV
It gets even better! If you want some more entertainment and all previously mentioned functions - nVidia TV is a great thing to get. You can use it for playing games and watching movies from your NAS. The greatest thing is - shield TV does video transcoding. So even if you have a cheap NAS with a weak CPU, in a combination of shield TV you can now automatically on the fly change video file size and format for a destination device. It is a great way to reduce network load and let older devices to play video content which normally was not supported on the device.