Ok this is weird. It went through the whole process of rebuilding the raid, when it'd finished I rebooted and then even though the logs showed that the raid was healthy the raid screen showed no raid had been configured.
I encountered something similar: After adding a 2nd disk in JBOD everything seemed to be ok. But, after a restart I noticed that the leftmost led kept blinking red! I was able to access the data but something was definitely wrong. Had a look into directory /var/log where I found file information with message "... JBOD resize fail(umount fail)". There wasn't anything mentioned in the log of the GUI . As I had backed up all my data before adding the 2nd disk I decided to play it safe. I destroyed the existing Raid and rebuilt it again.