1. One day, I've found the one of the mirror pair got disk error
2. Add spare (the box has four slots, and two disk were in the slot)

     (disk label as the same as others)
     raidctl -a /dev/wd2a raid0
   

3.   raidctl -F /dev/wd0a raid0
   

   This starts process to add spare disk
4. It took 30 days to complete spare setup
5. I was hesitating what to do for several days
6. One day the box does not give any response
7. Reboot
8. Asking root device: wd0a
   (and stopped)
9. (force to shutdown)
10. Remove suspicious unit
11. Boot ... fine

u# /sbin/raidctl -s raid0
Components:
           /dev/wd1a: optimal
           /dev/wd0a: optimal
No spares.
Component label for /dev/wd1a:
   Row: 0, Column: 0, Num Rows: 1, Num Columns: 2
   Version: 2, Serial Number: 377401575, Mod Counter: 512
   Clean: No, Status: 0
   sectPerSU: 32, SUsPerPU: 1, SUsPerRU: 1
   Queue size: 100, blocksize: 512, numBlocks: 3907029088
   RAID Level: 1
   Autoconfig: Yes
   Root partition: Soft
   Last configured as: raid0
Component label for /dev/wd0a:
   Row: 0, Column: 1, Num Rows: 1, Num Columns: 2
   Version: 2, Serial Number: 377401575, Mod Counter: 512
   Clean: No, Status: 0
   sectPerSU: 32, SUsPerPU: 1, SUsPerRU: 1
   Queue size: 100, blocksize: 512, numBlocks: 3907029088
   RAID Level: 1
   Autoconfig: Yes
   Root partition: Soft
   Last configured as: raid0
Parity status: clean
Reconstruction is 100% complete.
Parity Re-write is 100% complete.
Copyback is 100% complete.