Adding drives to EX4100

My Cloud OS 3 Personal & Network Attached Storage My Cloud EX Series
3

Thanks. I understand that in a RAID5 array I can replace one disk at the time and rebuild the array, but am wondering that in my situation a better solution would be to get the 4 bay unit, and run two drives in RAID1.
Then in the future if it looks like I need more space then I can get another two and change to RAID 10 mode. Would that be a viable and easy way to double my storage? I know that in 5 mode I get more actual space to use, but there is also the rebuilding issue.