First: you are probably having errors in your array. You need to check its status, immediately.

Example:

root@locutus:~# mdadm --detail /dev/md0

/dev/md0:

        Version : 1.2

  Creation Time : Fri Jun  1 15:30:31 2012

     Raid Level : raid10

     Array Size : 1953119232 (1862.64 GiB 1999.99 GB)

  Used Dev Size : 976559616 (931.32 GiB 1000.00 GB)

   Raid Devices : 4

  Total Devices : 4

    Persistence : Superblock is persistent



    Update Time : Sun Nov 25 21:39:44 2012

          State : clean 

 Active Devices : 4

Working Devices : 4

 Failed Devices : 0

  Spare Devices : 0



         Layout : near=2

     Chunk Size : 512K



           Name : locutus:0  (local to host locutus)

           UUID : b81ac63a:51a9b01c:c01812ec:86c534c4

         Events : 182



    Number   Major   Minor   RaidDevice State

       0       8        1        0      active sync   /dev/sda1

       1       8       49        1      active sync   /dev/sdd1

       2       8       17        2      active sync   /dev/sdb1

       3       8       33        3      active sync   /dev/sdc1

You will very likely find that you have errors... those will need to get addressed.

Moving on, you should change the way your fstab is set up, to use UUID instead of using devicename. Use the blkid command on your array (or a partition under your array) to get its UUID, then use that UUID to mount it in /etc/fstab.

Example:

root@locutus:~# blkid /dev/mapper/vg0-root

/dev/mapper/vg0-root: UUID="61998221-7b39-49cd-83f7-62fda973218c" TYPE="ext4" 

With this information, I would set my /etc/fstab entry for root to look like this:

UUID=61998221-7b39-49cd-83f7-62fda973218c    /    ext4    defaults,errors=remount-ro    0    1

Once you've done this, your system will find your partitions no matter whether their device names change or not (for example, if drive cables get swapped around).

You should also set up your /etc/mdadm/mdadm.conf to find things automatically without needing any particular drive to be in any particular location:

# mdadm.conf

#

# Please refer to mdadm.conf(5) for information about this file.

#



# by default (built-in), scan all partitions (/proc/partitions) and all

# containers for MD superblocks. alternatively, specify devices to scan, using

# wildcards if desired.

#DEVICE partitions containers



# auto-create devices with Debian standard permissions

CREATE owner=root group=disk mode=0660 auto=yes



# automatically tag new arrays as belonging to the local system

HOMEHOST <system>



# instruct the monitoring daemon where to send mail alerts

MAILADDR root



# definitions of existing MD arrays

ARRAY /dev/md/0 metadata=1.2 UUID=c913486a:e62c7ea1:cfb98b6b:253d1f62 name=lucas.mgscreativa.com.ar:0

NOTE THAT THERE ARE NO DEVICES SPECIFIED IN THIS CONF FILE. This is a feature, not a bug. With mdadm.conf set up this way, your system will automatically scan all attached drives looking for anything that belongs to your array, and will assemble them all as /dev/md0.