The calulator says if you start off witn one 2TB disk you only get aprox 1TB available space. Is it so? How can half of one single disk be used for safety when there is no other disk to spread the data on?
I was hoping it would be like RAID 0 with only one disk and if I pop in another I wont get any more space - just safety. When I pop in the third, available data will grow.
with only one disk you were hoping it would be like a basic volume (nothing like raid 0)
it does say in their knowledge base that if you only have a single disk it mirrors that data on that one disk (to try and protect against bad sectors). so your 1Tb of data will be written to the disk twice 
when you pop in a second disk your available space will double 
Ok, so the Calculator is right then.
yes