grep '/dev/md' /proc/mounts
and see if it says where it's mounted. If not, see what cat /proc/mdstat
says. Once you've unmounted it, fsck should work. mdadm -D /dev/md4
says - if it gives a dodgy disk, remove it mdadm -v /dev/md4 -r /dev/whatever
, replace it or fdisk it, then mdadm /dev/md4 -a /dev/whatever