When a new device appears (such as a new md array), udev springs in to
action and examines it to see if it should do something with it.
While udev (or some tool that it ran) is examining the md array it looks like
it is busy so an attempt to stop it will fail.
This sounds like a reasonable explanation to me, but I've not verified
this or looked into it further. I only mention the bug because the
behaviour sounds familiar and may be related.
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