If an array is being managed by mdmon, then just
write "inactive" to stop it, and let mdmon do the
final "clear". This makes sure mdmon has a chance
to read the final state and update the metadata properly.
After writing "inactive" with use "ping_monitor" to synchronise
with mdadm, then STOP the array just in case it is still running,
else we will get into an infinite loop in "mdadm -Ss".