Ok so your existing Disk were not formatted inside of the My Cloud which means the swap space is probably smaller than 2 GB. My Cloud OS 5 does not support < 2 GB swap space. You can verify by enabling ssh and running the following command.
cat /proc/partitions |grep sd*
If the swap space on your drive is < 2 GB, the following article includes the solution.
At any case, collect the system logs, send to support and let me know the case number.