Maybe you also want to reconsider using RAID. You could also use two seperate HDD and use rsync to copy the data from one HDD to the other one. By this you can create snapshots and keep different versions of files.
The OMV box will only be a backup for my Freenas file server. Nothing more nothing less. Snapshots tasks on that box are running as we "speak". I will use rsync with push tasks from the Freenas box to the OMV box for backup purposes. The reason I use a mirror is simple. This box will be situated at another location (of site backup). And when (and not if) a disk fails, a raid buys me a fair change to replace it before the other one fails. And besides of that, at a certain point I see the need for more space anyway and replacing mirrored disks one at a time with bigger ones is more convenient to me then starting all over.
So as soon as I have enough trust in this thing and have run the initial jobs (otherwise it would take forever) it will be relocated. I am lucky that a friend of mine is good in all the internet/network stuff and he will be helpful with the configuration and the needed services etc.
Thanks for sharing your thoughts on this subject with me.