[TriLUG] grub2-install fails with error: unknown filesystem

[Lee Fickenscher via TriLUG]

Greetings luggers,

I've been fruitlessly googling the issue over the last couple of days and
I'm hoping that someone has encountered and triumphed over this issue...

Issue:
Trying to install grub to the mbr of one of a dmraid1 mirror fails
with error: unknown filesystem.

The preamble:
We have a CentOS7 server with OS disks in mdraid1 partitioned thusly:

--------------------------------------
$ sudo fdisk -l /dev/sda

Disk /dev/sda: 500.1 GB, 500107862016 bytes, 976773168 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
Disk label type: dos
Disk identifier: 0x00000000

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1            2048    67110911    33554432   fd  Linux raid
autodetect
/dev/sda2   *    67110912    69208063     1048576   fd  Linux raid
autodetect
/dev/sda3        69208064   976773119   453782528   fd  Linux raid
autodetect
--------------------------------------

sda1 is swap
sda2 is /boot
sda3 is LVM root

During setup we make sure that the MBR is written on both raid members so
we can always boot and previously haven't had a problem.
We had a drive fail so the drive was replaced, partition table written, and
raid arrays synced. All was good until I tried to write the MBR.

Fix attempts:
I've gone through untold iterations of grub2-install commands, mainly
trying different modules and what not.
The only clue I've gotten is when running grub2-install with --debug which
provides:

grub2-install: info: /dev/md125 is not present.
grub2-install: error: unknown filesystem.

However, md125 is the /boot array and it's definitely there so I'm thinking
this may be a red herring.

I'm tempted to just use --force since it seems to be checking unrelated
filesystems but was hoping someone with more grub experience would chime in.

Thoughts?

Thanks!
-Lee