Ok, so maximum 8tb expansion, no matter the start or ending point, I didn't know this was the limitation. Good to know, although a bit strange (can anyone explain why, and might this change with future firmware upgrades?) But it definitely means I will want to start with 5 drives, so i can get the max out of the expansion limit.
Oh well, it just sucks that you set yourself up for playing with the new stuff, and then you cannot. I will wait for the replacement, and do as first planned! Thank you for your patience.