RAID1 is DEGRADED, but which disk is faulty?

[jesse reynolds]

Hi

How do you tell which disk has problems in a hardware RAID1 array? My 
machine has been up for nearly six months, and only the last three 
weeks of messages logs are retained. There is nothing any in the last 
three weeks of system logs about any kind of hardware error, or 
warning, or anything mentioning ar0, ad4, or ad6.

atacontrol is showing a DEGRADED status of the mirror:

# atacontrol status ar0
ar0: ATA RAID1 subdisks: ad4 ad6 status: DEGRADED


How does one tell which disk is faulty? Or is it possible that 
they've become out of sync and need rebuilding, in which case which 
one do I detach and reattach?

I'm scared to reboot as it's a running server. What can I do????

Cheers

Jesse

PS:

FreeBSD 4.6.2-RELEASE

atapci1: <Promise ATA100 controller> port 
0xdc00-0xdc3f,0xe000-0xe003,0xe400-0xe407,0xe800-0xe803,0xec00-0xec07 
mem 0xdffe0000-0xdfffffff irq 11 at device 16.0 on pci0

ar0: 76293MB <ATA RAID1 array> [9726/255/63] status: READY subdisks:
  0 READY ad4: 76319MB <ST380021A> [155061/16/63] at ata2-master UDMA100
  1 READY ad6: 76319MB <ST380021A> [155061/16/63] at ata3-master UDMA100


# atacontrol list
ATA channel 0:
     Master:      no device present
     Slave:       no device present
ATA channel 1:
     Master: acd0 <LG DVD-ROM DRD-8160B/1.01> ATA/ATAPI rev 0
     Slave:       no device present
ATA channel 2:
     Master:  ad4 <ST380021A/3.19> ATA/ATAPI rev 5
     Slave:       no device present
ATA channel 3:
     Master:  ad6 <ST380021A/3.19> ATA/ATAPI rev 5
     Slave:       no device present