Thanks Brian, I currently don't have raid at all on my OMV NAS. I just use an rsync script to do daily/weekly/monthly to a 2nd drive in the nas. Given I am more worried about backup than quick fixes I figured that was a better option than raid. The only problem is that both data and the backup are in the same machine, so a PSU failure or other damage could take them both out. I do do a manual backup to a usb drive occasionally but that only happens when I remember so is far from idea.There is a particular reason for me; I run Windows 10 Pro as a host server for Microsoft Windows Home Server 2011 (WHS2011) which runs as a virtual guest under Hyper-V. The Windows 10 host server is also intended to run another virtual guest for domestic CCTV recording when I decide upon which IP cameras I need & when funds permit (I have my eye on a Sigma 150-600 zoom for my Nikon D750 so they may have to wait a while longer).
WHS2011 is very effective at automatically backing up multiple client PCs (we have several) and provides incremental backups. Windows 10 with storage spaces allows me to expand my server storage gradually as my needs grow. My HP Microserver used to run WHS2011 natively but was storage constrained so I virtualised it. I used to back it up to external USB2 HDD which were very slow.
If I wasn't committed to WHS2011 & I wasn't an IT tinkerer, I would probably have a pair of NAS devices acting as a mirrored pair, whether they were PCs running OMV or dedicated devices such as Synology or QNAP would be down to how easy it was to mirror them, preferably in real-time.
I am still undecided on whether to expand my backup to the cloud (going to be very slow with a 200GB of photos though...) or just put another NAS somewhere safe.