Hi everybody

TL:DR How many drives in which RAID makes sense for a new NAS build?

I hope this is interesting enough for you to read through and to write your opinion.
Thank you in advance.

My Synology NAS has a (1) dead HDD (2 x 4TB) so I have to get a new HDD. While fixing I'd like to upgrade to futureproof but my Syno NAS can't handle more than 4TB per HDD (according to spec).

That's why I'm thinking about a new one but a prebuild is somewhat expensive and I'd like to build the things by myself.
Because I'm rack mounting all new devices, I found a case (Fantec SRC-2012X07) that can hold up to 12 3.5" HDDs. The HDDs in mind are around 12TB, right now I don't have the data to fill even one HDD but as I said, I want to build for the future where we will have even more and bigger data stored.

Right now I backed up data roughly around 4TB, so I think 2x 12TB HDD in RAID 1 would be plenty, I'd prefer a RAID 5 with 3 drives to begin with. But as soon as I want to upgrade the space and add more drives, I need to backup all data before I can wipe the previous RAID and build the new one.

As an example how I understand it:
I have a RAID 1 with 2x 12TB (lets say 10TB in total storage), the space is used with 10TB of data.
To be able to build a RAID 5 with 3 drives I have to backup first those 10TB on another drive an then build the RAID 5 with the 3 drives. That means I need at this point 4 drives.

Is that correct? Is there a better way?

Thanks for reading through and I hope you can write me your opinions about this.

I live in Switzerland and I'm using www.digitec.ch most of the time because they have a pretty good filtering system.

Cheers