Problem with loading files onto second internal hard drive

Here is the story, I recently got a 12tb mycloud ex2 ultra nas external hard drive. The reason for this was my 8tb my book nas drive was almost full. I have setup the ex2 as raid0 to get the max space which ment (2) 5.49tb drives. I started to transfer my video folders from mybook to mycloud and everything was going okay until drive a was 99% full then I could not move anything else to mycloud. My question is there anything that I could do to say drive a is full stop trying to load in it and start to loading drive b.

So what raid should I set it up to so I can get the full 12TB.