Useful RAID commands :

# Conf mdadm
cat /etc/mdadm.conf

# Current md stats
cat /proc/mdstat

# detail on a MD device :
mdadm --query --detail /dev/md3

# detail on partition
mdadm --examine /dev/sda3

Complete with OVH RAID help

# Copy partition table from safe disk to new disk (for GPT partition - see fdisk -l)
sgdisk -R /dev/newdisk /dev/safedisk

# afterwards, you have to randomize the GUID on the new hard disk to ensure that they are unique (from [howto forge](https://www.howtoforge.com/tutorial/linux-raid-replace-failed-harddisk/))
sgdisk -G /dev/sdb

# check partitions are copied 
sgdisk -p /dev/safedisk
sgdisk -p /dev/newdisk

# Add each partition to RAID cluster :
mdadm --manage /dev/mdX2 --add /dev/newdiskX2
mdadm --manage /dev/mdX1 --add /dev/newdiskX1

# follow reconstruction with 
mdam --detail /dev/mdX
or 
cat /proc/mdstat
#  Rebuild Status : 21% complete