Ok, I did it 
My Mock now has 4 internal hard drives.
1 SSD for Roon and 3 for internal storage (2x 1TB and another 320GB I had laying around…)
What I found is that the web interface doesn’t support multiple storage drives so I had to connect each hard drive by itself in order to format it and have it included under the “storage” folder.
Somehow after adding each hard drive individually each one gets its own folder (internal storage 1, internal storage 2, etc.)
So it’s only the web interface which is half baked for multiple drives but the underlaying Linux build supports it.
Bottom line - up and running with all drives