Adding Disk and Changing configuration

Hopefully this is the correct forum for this.

On impulse I bought a Netgear ReadyNAS RN104 Diskless when I saw it on Amazon for £180, at the same time I bought one WD Red 3TB disk(The one on the HCL).

I didn't really consider buying any more as the plan is to become familiar with the interface and controls then reset to Factory Settings and start again, this time with more disks.

My partner however has decided she really likes the NAS and we are upgrading the space already, she has some data on it, not too much so I COULD copy the data off reset and replace the data, however I started thinking about what I'm actually planning on doing with it and I realised that I really just want a Raid 1 mirror of 2x3TB disks giving 3 TB(2.7 ish but you know what I mean) for hard to replace stuff like photos, documents and weekly pc backups and 2 stand alone volumes of 3TB, one for easily replaceable data like music and one for transient data like tv shows that are deleted once watched.

My question is, what would people with experience of OS6 do to achieve this?

This is not a question about raid levels.

I'm guessing I have to switch to Flex-Raid and specify it from diskless, but I'm also wondering if you can do it hot whilst saving the data already there.

Thanks for reading.
Glenn.