How to correctly install GRUB on a soft RAID 1?

If the two disks are /dev/sda and /dev/sdb, run both grub-install /dev/sda and grub-install /dev/sdb. Then both drives will be able to boot alone.

Make sure that your Grub configuration doesn't hard-code disks like (hd0), but instead searches for the boot and root filesystems' UUIDs.

I'm not aware of support in Grub to declare two disks as being in a RAID-1 array so that grub-install would automatically write to both. This means you'll need to run grub-install again if you replace one disk; it's one more thing to do in addition to adding new members to the RAID arrays.

I've done this on quite a few servers over the past while and found issues sometimes, usually what I do is run the following

sudo grub-install /dev/sdb
sudo update-grub /dev/sdb

The problems I've run into are usually displayed with error messages like

update-grub … update-grub failed with no such disk …

To resolve this i've run

sudo mv /boot/grub/device.map /boot/grub/device.map.old 
sudo update-grub 

and this seems to have resolved the issue - as a test in some cases I have removed /sdv/sda and just used /dev/sdb to boot from but this is not ideal and most likely not possible in most production environments!