Appreciate the response, but SMART isn't the issue here. As mentioned in my last post, all of the drives say SMART is unavailable. That's not what I'm asking about (only mentioned to give a complete picture). The problem is that one drive displays as Not Usable in Scanner, and this didn't happen previously on this drive (ie. it displayed the same as the other drives).
How do we troubleshoot the cause of the Not Usable status? The drive appears to be fine and I'm inclined to ignore the status, but I'd like to understand why it's happening so I can make sure there's not some underlying problem with the drive.