Principle:
If you look at the result of the command df -h and you are using the mdmadm software RAID 1 then you can only see the /dev/mdx partitions.
eg.
Filesystem Size Used Avail Use% Mounted on
/dev/md2 1.8T 524G 1.2T 31% /
udev 10M 0 10M 0% /dev
tmpfs 3.1G 288K 3.1G 1% /run
tmpfs 5.0M 0 5.0M 0% /run/lock
tmpfs 9.3G 0 9.3G 0% /run/shm
/dev/md1 496M 77M 394M 17% /boot
Since the command ‘grub-install /dev/md0’ or ‘grub-install /dev/md0’ produces an error, the main principle is to install GRUB on both drives that constitute the components of software RAID 1.
STEPS:
In order to allow any of the two hard disks to boot using GRUB the following line must be disabled in the file /etc/default/grub:
GRUB_TERMINAL=console
Now we need to register all the drives that constitute the software RAID drives.
grub-mkdevicemap -n
We find out which of the drives(not partitions) are used for the RAID
fdisk -l | grep 'Disk /dev/sd'
eg:
Disk /dev/sdb: 1.8 TiB, 2000398934016 bytes, 3907029168 sectors
Disk /dev/sda: 1.8 TiB, 2000398934016 bytes, 3907029168 sectors
We update the GRUB configuration:
update-grub
Finally we install GRUB on both of the above listed drives(/dev/sda and /dev/sdb)
grub-install /dev/sda ; grub-install /dev/sdb
Now each drive can be used to start the system.