I just spend the last couple hours working through the same error but with a single disk set up. Perhaps this will help.
Try running a LIVE USB/CD on the machine and mount sbc1the EPS partition. In there check out the loader/entries/arch.conf file. What I found is that I had a setting like root=PARTUUID=xxxxxxxx.... What I had to do was change it to root=/dev/mapper/<partition label>. This it worked as expected. Hope that helps.