Ok, removed both discs, moved right to left and left right bay empty, turned on and could access data.
So that means that only the (original) left disc was corrupt.
Questions now are...
1. Why? Corruption due to disc, DNS, or just plain bad luck?
2. Where to? If corruption was due to DNS I'd think a default was in order. If it wasn't, what would the correct process be to restore RAID1 - I'm thinking remove both, return corrupt disc only, format, remove, put in both discs and let it restore the RAID. Any other thoughts/ideas?