> Worse - if you created the array with an older version of mdadm, then added a spare with a newer version then it is possible that different devices have different data offsets. If that seems to be the case it would be best to ask for help on firstname.lastname@example.org as you'll need a special non-released version of mdadm to fix things up.
In fact, being able to specify the data offsets (at least at creation time) would be a very interesting feature when using metadata 1.1 or 1.2 with HD with specific alignment constraints. You can read more details in http://bugs.debian.org/614841
Can you consider adding such an option?