Hard drive failure during rebuilt - next steps?

Hi there,

I have just undertaken an upgrade of the hard drives in my ReadyNAS314 as follows:

* Bays 1 & 2 - 2 x 6TB WD Red Pro - to remain the same

* Bays 3 & 4 - 2 x 2TB WD Red Pro to 2 x 6TB WD Red Pro

I swapped out Bay 3-4 at which point the RAID started to rebuild. I've returned overnight to see a warning that the NAS is degraded and the drive in Bay 1 has disappeared from the 

What I really need help with is:

1. How can I check whether all my data is ok? 

2. How can I check whether this is a drive failure or issue with the NAS. The console displays the volume as missing (rather than corrupted) - image below

3. What to do next!

Many thanks

Richard