After booting, my RAID1 device (/dev/md_d0 *) sometimes goes in some funny state and I cannot mount it.

^{* Originally I created /dev/md0 but it has somehow changed itself into /dev/md_d0.}

# mount /opt
mount: wrong fs type, bad option, bad superblock on /dev/md_d0,
       missing codepage or helper program, or other error
       (could this be the IDE device where you in fact use
       ide-scsi so that sr0 or sda or so is needed?)
       In some cases useful info is found in syslog - try
       dmesg | tail  or so

The RAID device appears to be inactive somehow:

# cat /proc/mdstat
Personalities : [linear] [multipath] [raid0] [raid1] [raid6] [raid5] 
                [raid4] [raid10] 
md_d0 : inactive sda4[0](S)
      241095104 blocks

# mdadm --detail /dev/md_d0
mdadm: md device /dev/md_d0 does not appear to be active.

Question is, how to make active the device again (using mdmadm, I presume)?

(Other times it's alright (active) after boot, and I can mount it manually without problems. But it still won't mount automatically even though I have it in /etc/fstab:

/dev/md_d0        /opt           ext4    defaults        0       0

So a bonus question: what should I do to make the RAID device automatically mount at /opt at boot time?)

This is an Ubuntu 9.10 workstation. Background info about my RAID setup in this question.