Am 04.11.2017 um 19:30 schrieb Wols Lists:

mdadm did exactly what it is supposed to do. A mirror with one drive is
degraded, so it assembled the array AND STOPPED. Once you force it past
this point, I think it will happily go past again no problem, but it's
designed to refuse to proceed with a damaged array, if the array was
fully okay previous time.

So, in other words, the disk and everything else is fine.

What's happened is that mdadm has assembled the array, realised a disk
is missing, AND STOPPED.

why would it be supposed that a simple mirror with a mising disk is 
stopped while the whole point of mirroring is to not care about one of 
the disks dying?