If it works, I'd still wonder why md54 comes up fine. That entry
is also missing `level` and `num-devices`.
1st WAG ?
/dev/md/54 is 'just' raid1, without an atypical spare
raid5 & raid1+spare are less common.
whether autoassembly is smart enough to pick up the diffs without the explicit spec in
/etc/mdadm.conf?
again, dunno without trying.