The eMMC I bought for the XU4 is a 32 GB 5.0 which according to http://forum.odroid.com/viewtopic.php?f=135&t=18683
should work in the C2. I hope this is so but I will know in a week’s time. Was yours the same type?
Running Dietpi from the eMMC seemed faster than from the SSD but I don’t have figures to confirm this.
With regard to the Cloudshell display for the USB drive I decided to put a 16 GB 3.0 flash drive in the lower USB 3.0 port - the display shows the spare capacity on this instead of duplicating flash and USB storage. The flash drive used is to backup the SDD by rsync.
Again thanks for your prompt help with these problems.