It started responding very slowly when I logged in via SSH.
I looked at the running processes using top.
I thought that rebooting it would remedy it quickly.
But when I booted it up, the problem persisted.
The RAID warnings said a drive had overheated and failed.
So I ordered a new drive from Amazon.com.
I didn't pay for rush shipping but it arrived quickly.
I stuck the new drive in and rebooted the NAS.
It took a long time to rebuild the volume.
Now my NAS is error free and ready for hot rsync again.