Ubuntu18.10 installation on multi device Btrfs failed at installing bootloader

[drstoop]

Hi, I tried to install Ubunutu on a multi device Btfs file system. Before installation I created the Btrfs multi device system with the following command:
```
$ sudo mkfs.btrfs --force --data single --metadata single --label Ubuntu_Btrfs /dev/sda1 /dev/sdc1
$ sudo btrfs device scan
```
I checked the mount handle to be /dev/sda1 (& not /dev/sdc1) and tried to install Ubuntu with /dev/sda1 as root `/`. I suppose the error occurs due to the identical UUIDs of /dev/sda1 and /dev/sdc1 when trying to install the bootloader - but I don't know.

Motivation for this was possibly another Btrfs-incompatibility: After installing Ubuntu18.10 on the Btrfs-/dev/sda1 I btrfs-device-added /dev/sdc1 to /dev/sda1. This caused the boot process and the login screen to be very 'freezy' having issues to wait for the screen to build up and the cursor movement and keyboard input to finally display on the screen...

Thx & Greets!
Max

ProblemType: Bug
DistroRelease: Ubuntu 18.10
Package: ubiquity 18.10.12
ProcVersionSignature: Ubuntu 4.18.0-10.11-generic 4.18.12
Uname: Linux 4.18.0-10-generic x86_64
ApportVersion: 2.20.10-0ubuntu13
Architecture: amd64
CasperVersion: 1.399
Date: Sat Jan 26 17:50:05 2019
InstallCmdLine: file=/cdrom/preseed/ubuntu.seed boot=casper initrd=/casper/initrd quiet splash --- maybe-ubiquity
LiveMediaBuild: Ubuntu 18.10 "Cosmic Cuttlefish" - Release amd64 (20181017.3)
SourcePackage: grub-installer
UpgradeStatus: No upgrade log present (probably fresh install)