mdadm --manage /dev/md0 --fail /dev/sdb1
remove the disk :
mdadm --manage /dev/md0 --remove /dev/sdb1
change the physical drive, copy the partition, create the exact same partitioning as on/dev/sda :
sfdisk -d /dev/sda | sfdisk /dev/sdb
add the new drive
mdadm --manage /dev/md0 --add /dev/sdb1
done, check the progress :
cat /etc/mdstat
references: