The Cell hardware has only been tested with hard drives up to 8TB. In fact, a low-energy board like the one inside the Cell may start malfunctioning managing more storage due to hardware limitations.
In order to be safe and not have users connect an excessively large drives with unknown results, we have decided to put a boundary at 8TB of physical storage, corresponding to 4TB of cloud space. This means that regardless of how much physical storage you provide to the Cell, you will not be given more than 4TB of cloud space. This will help in keeping the network stable and efficient.
In the future, if we have evidence and data that the Cell is stable with more than 8TB of external storage attached per Cell, we will raise the limit. Right now the best way to you have to go beyond 4TB is to use another Cell.