On Friday, March 14th the warning indicator on Merlin2 was flashing, and the third drive was also flashing yellow. As we were in the middle of the data move from Merlin to Merlin2, we decided to push onward and deal with the problem later.
Today, the OS went BSOD. The server was shut down, then powered back up. The server came back up without any data loss and automatically started rebuilding the array onto the previously failed disk.
We will keep an eye on this. We assume that there is some fault in the server, either in the RAID controller, the SCSI backplane, or the main system board.