Thanks for the answer. Do you have any pointers to how to identify from device name, which physical hard disk it is? For example, I have a RAID1 mirror set, the devices are disk1s3 (failed) and disk4s2. These are partitions on 2 different external disks. How do I know which one disk1s3 is? Is there a way to reconcile it with the hardware?
I suppose I could unplug one of the disks and see what happens, but that would be disruptive and extreme. What would happen to the RAID set, what if Time Machine is running to the mirror?
diskutil info diskX
where X is the number of the failed disk. This will get you a device media name. If you use -plist flag you can also get the device tree information. As for serial number for the drive, you'll need to use system_profiler which contains both the bsd name and the serial number for each drive.