When booting after upgrade from Debian Lenny to Squeeze with a working 4.0TB external usb Raid0 the system no longer recognised the Raid0 md at boot up.

I added the following command to /etc/rc.local as a work-around but cannot figure out why the boot process no longer recognises md0.

# from /etc/rc.local
...
#
# build the raid from component devices
mdadm --assemble -v /dev/md0 /dev/sda1 /dev/sdb1
#
# set logical volume to active
vgchange -a y /dev/ns2vg00
#
# mount md0 raid device
mount -t ext4 /dev/ns2vg00/stor /stor
#
...

Does anyone have an idea what the problem might be in the boot process?