dmraid -r
/dev/sdc: pdc, "pdc_bbddhiahc", mirror, ok, 781291648 sectors, data@ 0
/dev/sdd: pdc, "pdc_bbddhiahc", mirror, ok, 781291648 sectors, data@ 0

 dmraid -s
*** Set
name   : pdc_bbddhiahc
size   : 781291648
stride : 128
type   : mirror
status : ok
subsets: 0
devs   : 2
spares : 0

mdadm.conf:
DEVICE /dev/sdb1 /dev/sdc1 /dev/sdd1
ARRAY /dev/md0 name=var super-minor=0 devices=/dev/sdb1,/dev/sdc1,/dev/sdd1 level=5 auto=yes

On boot, the device is not assembled.
I have to mdadm -A /dev/md0 /dev/sdb1 /dev/sdc1 /dev/sdd1 before it gets assembled.

Is the reason that /dev/sdb doesn't exist as far as dmraid is concerned?