Graças a Terrance, encontrei este tutorial que funciona muito bem
basicamente as instruções são as seguintes:
1.) Change the partition type --> fdisk /dev/sdd --> press t, select partition, l to list codes, looking for "Linux raid setup" usually fd
2.) Do the above for every volume you plan to add --> fdisk /dev/sda | t | 1 | fd
3.) Zero the superblock of the devices --> mdadm --zero-superblock /dev/sdd4
4.) Create the raid device --> mdadm --create /dev/md0 --level=1 --raid-disks=2 /dev/sdd4 /dev/sda1
5.) check to make sure device exists --> cat /proc/mdstat --> should now see your raid device
6.) Create filesystems on our raid array --> mkfs.ntfs /dev/md0
7.) Mount raid array --> mount /dev/md0 /srv
8.) I recommend a benchmark for your array so you have an idea if something is going wrong