Ok you've restarted the server as the drive references have changed
The drives in the array are currently [cdef] sdd and sdf are showing as spares, this is one of the issues, the second is #9 the error -> no uptodate device for slot 1 and the same for slot 3
Going back to your initial post where you stated Steam reported disk errors, this could be i/o errors which could be related to hardware issues where the data being copied is experiencing intermittent write problems to the drive/s.
This could be either the drive/s themselves or the connectivity of the drive, sata cable, power cable, backplane (backplane is a drive bay where a drive is plugged in i.e. 4 port drive bay these typically have/use a backplane.
Can you give some info in relation to how the drives are connected, this might be a hardware issue.
But I don't get it... Why
If this is hardware related it would explain why it 'just happened' if it's drive related one would have expected SMART issues warning of a possible problem