Thanks for the responses and sorry for the delay in mine, but my duties as new father are no joke.
What I had in mind was something like:
1. Today I buy two HC-2 with one 2TB HDD each one and set them up in RAID-1. My thought here was that in this way I will hardly lose data. When one disk dies I can substitute it and I will still have all the info in the other disk. (Wife's daily photos are critical, you know).
2. I may use a USB attached big HDD to store incremental backups from time to time.
3. Tomorrow, when I need more space I buy the same hardware again and attach it to the existing setup. My thought here is that in this way I could expand seamless and unexpensively my setup and not have to "throw away" anything.
4. Repeat step 3.
So from your answers I take that this is not possible to do this with only the HC-2s and OMV, right? Is it just a bad idea? What would you do to achieve something similar?
What I am thinking now is having those 2*X HC-2s share their drives on the network with NFS and have let's say an Odroid-C2 be the OMV and glues it all up together. Would this be a nice approach?