mdadm frozen and doesn't realize array is dead/missing failed due to unplugged drives
This was a surprising bug but I unplugged all drives for an array md127. At first it was just 1 drive and mdadm seemed to notice this. I unplugged the second drive taking the array offline but mdadm did not realize it was offline and still showed a non-existent disk as being part of it. This created problems trying to unmount it or even to stop this array with mdadm freezing.
As for how to fix it I can only think of making sure you are not in a mounted path of........
mdadm how to add additional RAID 1 drives
This is a great way to upgrade your RAID array or move it/copy it to a new set of hard drives.
Eg. you have a current RAID 1 array on older/slower drives.
Just add at least 1 of the new drives to the array, update grub/install it and then boot into it. Then you have a transparent data migration that is fully synchronized.
mdadm --grow /dev/md126 --raid-devices 3
md127 : active raid1 sdc1........