Yeah, I'm not a fan of EFI because:
Both sda and sdb are currently identical.
/ is /dev/md125 which is sda3 and sdb3
/boot is /dev/md126 which is sda4 and sdb4
/boot/efi is only on /dev/sda1
and IIUC, can't be raided which means you have to remember
to sync /dev/sda1 to /dev/sdb1 after anything that updates /dev/sda1.

And if you could raid /boot, I don't see a need for it to be a
separate partition.  My /boot is on /.

I just buy a couple of 2TB (or less) disks where / and swap sit
and boot legacy BIOS.  If I need more storage, I'll get a couple
of 4TB (or larger) disks and partition them:
[0:root@elmo vhosts 2]$ fdisk -l /dev/sdc
Disk /dev/sdc: 3.7 TiB, 4000787030016 bytes, 7814037168 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disklabel type: gpt
Disk identifier: 72FCBBCF-6AA6-47F4-8A99-965F773B35CB

Device        Start        End    Sectors  Size Type
/dev/sdc1      2048   31250431   31248384 14.9G Microsoft basic data
/dev/sdc2  31250432 7814035455 7782785024  3.6T Microsoft basic data

... and what's with this 'Microsoft basic data' stuff. /dev/sdc1 is swap
and /dev/sdc2 is ext4.

I guess I woke up grumpy this morning.  :-)

Bill



On 3/16/2018 5:55 AM, Gary Stainburn wrote:
On Thursday 15 March 2018 16:34:26 Bill Shirley wrote:
Run:
grub2-install /dev/sda
grub2-install /dev/sdb

Now, you can boot from either drive.

Bill

Hi Bill,  Thanks for this.  However, when I try this I get:

[root@lou ~]# grub2-install /dev/sda
/usr/lib/grub/x86_64-efi doesn't exist. Please specify --target or --directory
[root@lou ~]# 

A quick google shows gives the indication that this is for MBR systems where 
mine is UEFI. 

A slower google hasn't given me instructions on how to do this on a UEFI 
system in a way that I can understand.

Can anyone give the equivelant to Bill's commands above.

To review the situation. I have sda which I boot from. I have a brand new sdb 
which I use for RAID1. Should sda fail at some point I want to be able to 
unplug sda, and be able to boot from (what is currently) sdb.

this is what the two drives both look like at the moment,

[root@lou ~]# parted
GNU Parted 3.1
Using /dev/sda
Welcome to GNU Parted! Type 'help' to view a list of commands.
(parted) print                                                            
Model: ATA ST3000DM001-1CH1 (scsi)
Disk /dev/sda: 3001GB
Sector size (logical/physical): 512B/4096B
Partition Table: gpt
Disk Flags: 

Number  Start   End     Size    File system  Name                  Flags
 1      1049kB  53.5MB  52.4MB  fat16        EFI System Partition  boot
 2      53.5MB  2837GB  2837GB                                     raid
 3      2837GB  2942GB  105GB                                      raid
 4      2942GB  2994GB  52.5GB  ext4                               raid
 5      2994GB  3001GB  6296MB                                     raid
 6      3001GB  3001GB  1049kB                                     bios_grub

(parted)            
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