From the name, it looks like it was used for some migration work.
check whether the file system is still in use
$ fuser /migwork
No process is using /mitwork, so we can safely remove it.
$ umount /migwork
update /etc/fstab to prevent it from being mounted after reboot.
#/dev/md20 /migwork ext3 defaults 1 2
check the information about md20
$ cat /proc/mdstat
$ mdadm --detail /dev/md20 | sed -n '/RaidDevice/,$p'
$ mdadm --stop /dev/md20
$ mdadm --remove /dev/md20
mdstat showed these sub disks are from SAN, inform storage engineer to take them back.