system identifier light blinking

This xserve3,1 (dual 2.93GHz, 24 Gig RAM, 3x 2TB SAS drives in bays, no hardware RAID card) has been in service for a while, and last Saturday, I was tipped off to a problem because of the blinking of the system identifier light - 3 or 4 rapid blinks (hard to tell), followed by a pause, and repeat. At the time, it was running Lion (and Server); Disk Utility told me that one of the 3 internal drives, which are mirrored, had failed. So I removed the drive, updated to El Capitan (and Server), and replaced the drive with a new one, demoted the failed drive, and added the new drive to the set. It rebuilt just fine, so I'm back to 3 drives, mirrored. But surprise, the blinking is still going on.

I bought this machine used, and it did not come with any disks. When I had my xserve2,1, it came with a disk that I could use to install some software to boot the server off of, and test the server that way - but no disk with this machine.

Here's my question: How can I find out what fault is present? Can Server software do anything to help me? I've looked at alerts & logs, but don't really know what to look for; there weren't any obvious entries...

Yeah, it's an old machine, but I'm hoping someone can give me some ideas.

TIA,

Andrew