DELL CERC amr RAID card beeping, dead drive? how to diagnose/fix?

I have a DELL 600SC which came with a DELL CERC RAID controller.  It's
recognized by FreeBSD-CURRENT as an "amr" device even though it's got
four ATA disk channels on it instead of the documented SCSI drives for
the PERC controller.  I have 4x WD1200JB ATA 120GB disks on it which
have been running fine for a few months as a set of RAID-5 volumes.
>From "dmesg":

  amrd0: <LSILogic MegaRAID logical drive> on amr0
  amrd0: 9999MB (20477952 sectors) RAID 5 (optimal)
  amrd1: <LSILogic MegaRAID logical drive> on amr0
  amrd1: 111093MB (227518464 sectors) RAID 5 (optimal)
  amrd2: <LSILogic MegaRAID logical drive> on amr0
  amrd2: 111093MB (227518464 sectors) RAID 5 (optimal)
  amrd3: <LSILogic MegaRAID logical drive> on amr0
  amrd3: 111099MB (227530752 sectors) RAID 5 (optimal)

An hour ago, it started beeping at me.  I suspect this is the CERC
card warning me that one of the disk drives has failed and that I'd
better do something about it. :-(

Is there a way to diagnose it from a live system, to query which of
the four ATA drives it thinks is dead, so I can replace it?

(Seems to me that a WD1200JB drive should last a lot longer than a few
months it's been running, in a properly ventilated DELL box; any ideas?)

Anyone have experience with this CERC controller and replacing a
drive?  My biggest fear is that I haven't tested the RAID rebuild and
that even when I do replace the failed (?) drive it won't do the
automatic rebuild and save my data.

Other suggestions?

Thanks.