ubuntu RAID 5 or 6 with missing drive(s)

some RAID configurations allow missing drives, for example, 5 or 6 or up.

say, raid 5 with 1 drive missing:

    $ cat /proc/mdstat

if mdadm drive is detected but inactive, stop it.

    $ sudo mdadm --stop /dev/md5

indicate using only 2 drives for read only.

    $ sudo mdadm --assemble --readonly /dev/md5 /dev/sdc1 /dev/sdd1

first time likely got error, force it

    $ sudo mdadm --run /dev/md5

    $ ls /dev/md5 

now exist

    $ sudo mount /dev/md5 /mnt

drive now readable.

when done

    $ sudo umount /mnt

    $ sudo mdadm --stop /dev/md5