How do I replace a disk marked as removed from a linux md raid-5 array?
Run the following commands on the removed device:
mdadm --zero-superblock /dev/sdXn
mdadm /dev/md0 --add /dev/sdXn
The first command wipes away the old superblock from the removed disk (or disk partition) so that it can be added back to raid device for rebuilding. Make sure that you run this command on the correct device!!