On 04/11/17 20:36, Reindl Harald wrote:

quoted

While the system is RUNNING, yes. But if the array is STOPPED, mdadm
will refuse to start it. At least, that's certainly how I understand
it works ...

Do you REALLY want the system to be running, and give you no clue that
it's not working properly?

mdmon typically writes a mail about a degraded array - so far away from
"no clue"

mdmon? What's that? Yes, I know, it's the monitor. But how do you know
whether or not it's running? It certainly isn't on my system.

Anyways, as I said, as far as I am aware, "mdadm --run" does NOT work on
a degraded array unless either the previous --run was on a degraded
array, or you use --force.

Which means if you remove a disk from an array, and then try to restart
the array, the restart will fail. Which is exactly the OP's scenario.

Cheers,
Wol