RAID1 /dev/md4 lost one disk partion /dev/sdb4 after reboot, WHY??

[Bruno Wolff III]

On Sun, Mar 29, 2009 at 20:31:53 +1100,
  L <yuanlux at gmail.com> wrote:
> 
>  any suggestion??

Look through your log files to see why its being dropped from the array.
There may be bad blocks or something.

Run a smartctl long test on the disk to see if that picks up any bad blocks.
If you find one, you can boot up in rescue mode and write a copy from the
good disk. If you find a bunch you might be seeing indications that your
disk is failing.