Why is Grub not installed when installing Debian 9 on RAID1?

Question

Debian

Hello!
I put Debian 9 on a SuperMicro server (without a hard controller), during installation I assemble two SATA HDDs (8Tb) into RAID1 using the debian installer (I created a Physical RAID partition on both disks, assembled RAID1 from them, but created ext4 in the resulting raid, specified the mount point / ).
Further, at the end of the system installation, I choose to install the system loader on / dev / sda - I get

Failed to install GRUB on /dev/sda.
Failed to execute command 'grub-install /dev/sda'
This is a fatal error.

The operation is routine (I have more than one server with Debian 9 on top of software-raid in production), I don’t understand what’s wrong.
In the BIOS, the SATA settings were originally set to AHCI, changed to IDE Compatible - the result is the same.
UPD: solved the problem like this
In the BIOS, I chose BIOS Legacy,
On both disks I created a 200MB biosgrub partition, the rest of the space was allocated for the Physical RAID volume,
I assembled RAID1, installed the system on it,
Installed it on / dev / sda GRUB using the installer - successfully,
Booted into system, installed GRUB on /dev/sdb,
Uncommented GRUB_TERMINAL=console in /etc/default/grub, Executed
update-grub.
Now I have a system on software-RAID1 and the bootloader is installed on both drives.

Answer 1

Try to boot into rescue, mount and install manually, before that change fs to ext3 in the boot partition

Answer 2

At you raid gathers already after start of OS. When bootloader starts, there is no such device. It must be assembled by the kernel. The output is a small screw (flash drive) with /boot/ placed there. In his MBR and put a hornbeam.

Answer 3

Just the day before yesterday I installed Debian 9.5 on soft-raid. True, I did not select /boot/, and rolled over mdraid LVM.
Perhaps it will be enough not to allocate a separate /boot,
the bootloader put it in /dev/sda, then, on a live system, it also installed it in /dev/sdb.