Volume expansion failed-many times

I have not done a factory reset at all. The first build (at factory default) was 3X3TB, and the only addition was the 3X3TB extra. At the time of the addition I did not realise that I would max out at 8TB addition, but I do now. The problem that I am having is after only 1X3TB. On the smart logs, each disk says "SMART overall-health self-assessment test result: PASSED" & "SMART Error Log Version: 1 No Errors Logged". I should say that the Western digital scan showed 4 partitions - 2 very tiny ones, but 2 good sized ones. I assumed that these two main partitions were where the ReadyNas was doing the expansion. If that is so, then it was about 25-30% through the disk.